diff --git a/core/include/traccc/finding/details/find_tracks.hpp b/core/include/traccc/finding/details/find_tracks.hpp
index 6004270c8..f24cc28f6 100644
--- a/core/include/traccc/finding/details/find_tracks.hpp
+++ b/core/include/traccc/finding/details/find_tracks.hpp
@@ -241,7 +241,7 @@ track_candidate_container_types::host find_tracks(
                 // Run the Kalman update on a copy of the track parameters
                 const bool res =
                     sf.template visit_mask<gain_matrix_updater<algebra_type>>(
-                        trk_state, in_param, false);
+                        trk_state, in_param);
 
                 // The chi2 from Kalman update should be less than chi2_max
                 if (res && trk_state.filtered_chi2() < config.chi2_max) {
diff --git a/core/include/traccc/fitting/actors/surface_id_aborter.hpp b/core/include/traccc/fitting/actors/surface_id_aborter.hpp
new file mode 100644
index 000000000..68fa11a86
--- /dev/null
+++ b/core/include/traccc/fitting/actors/surface_id_aborter.hpp
@@ -0,0 +1,41 @@
+/** TRACCC library, part of the ACTS project (R&D line)
+ *
+ * (c) 2024 CERN for the benefit of the ACTS project
+ *
+ * Mozilla Public License Version 2.0
+ */
+
+#pragma once
+
+// Project include(s)
+#include "detray/definitions/detail/qualifiers.hpp"
+#include "detray/propagator/base_actor.hpp"
+
+// System include(s)
+#include <limits>
+
+namespace traccc {
+
+/// Aborter triggered when the next surface is reached
+struct surface_id_aborter : detray::actor {
+    struct state {
+        detray::geometry::barcode m_abort_id;
+    };
+
+    template <typename propagator_state_t>
+    DETRAY_HOST_DEVICE void operator()(state &abrt_state,
+                                       propagator_state_t &prop_state) const {
+
+        auto &navigation = prop_state._navigation;
+
+        // Abort if the propagator is on the surface with abort ID
+        if ((navigation.is_on_sensitive() ||
+             navigation.encountered_sf_material())) {
+            if (navigation.barcode() == abrt_state.m_abort_id) {
+                prop_state._heartbeat &= navigation.abort();
+            }
+        }
+    }
+};
+
+}  // namespace traccc
diff --git a/core/include/traccc/fitting/kalman_filter/gain_matrix_updater.hpp b/core/include/traccc/fitting/kalman_filter/gain_matrix_updater.hpp
index 2dd6b4ef2..b76bf2837 100644
--- a/core/include/traccc/fitting/kalman_filter/gain_matrix_updater.hpp
+++ b/core/include/traccc/fitting/kalman_filter/gain_matrix_updater.hpp
@@ -40,19 +40,16 @@ struct gain_matrix_updater {
     TRACCC_HOST_DEVICE inline bool operator()(
         const mask_group_t& /*mask_group*/, const index_t& /*index*/,
         track_state<algebra_t>& trk_state,
-        const bound_track_parameters& bound_params,
-        const bool backward_mode) const {
+        const bound_track_parameters& bound_params) const {
 
         using shape_type = typename mask_group_t::value_type::shape;
 
         const auto D = trk_state.get_measurement().meas_dim;
         assert(D == 1u || D == 2u);
         if (D == 1u) {
-            return update<1u, shape_type>(trk_state, bound_params,
-                                          backward_mode);
+            return update<1u, shape_type>(trk_state, bound_params);
         } else if (D == 2u) {
-            return update<2u, shape_type>(trk_state, bound_params,
-                                          backward_mode);
+            return update<2u, shape_type>(trk_state, bound_params);
         }
 
         return false;
@@ -61,8 +58,7 @@ struct gain_matrix_updater {
     template <size_type D, typename shape_t>
     TRACCC_HOST_DEVICE inline bool update(
         track_state<algebra_t>& trk_state,
-        const bound_track_parameters& bound_params,
-        const bool backward_mode) const {
+        const bound_track_parameters& bound_params) const {
 
         static_assert(((D == 1u) || (D == 2u)),
                       "The measurement dimension should be 1 or 2");
@@ -92,10 +88,8 @@ struct gain_matrix_updater {
             bound_params.covariance();
 
         // Set track state parameters
-        if (!backward_mode) {
-            trk_state.predicted().set_vector(predicted_vec);
-            trk_state.predicted().set_covariance(predicted_cov);
-        }
+        trk_state.predicted().set_vector(predicted_vec);
+        trk_state.predicted().set_covariance(predicted_cov);
 
         if constexpr (std::is_same_v<shape_t, detray::line<true>> ||
                       std::is_same_v<shape_t, detray::line<false>>) {
@@ -138,59 +132,12 @@ struct gain_matrix_updater {
         }
 
         // Set the track state parameters
-        if (!backward_mode) {
-            trk_state.filtered().set_vector(filtered_vec);
-            trk_state.filtered().set_covariance(filtered_cov);
-            trk_state.filtered_chi2() = matrix_operator().element(chi2, 0, 0);
-
-            // Wrap the phi in the range of [-pi, pi]
-            wrap_phi(trk_state.filtered());
-        }
+        trk_state.filtered().set_vector(filtered_vec);
+        trk_state.filtered().set_covariance(filtered_cov);
+        trk_state.filtered_chi2() = matrix_operator().element(chi2, 0, 0);
 
-        if (backward_mode) {
-            assert(trk_state.filtered().surface_link() ==
-                   bound_params.surface_link());
-
-            const matrix_type<e_bound_size, e_bound_size> predicted_cov_inv =
-                matrix_operator().inverse(predicted_cov);
-            const matrix_type<e_bound_size, e_bound_size> filtered_cov_inv =
-                matrix_operator().inverse(trk_state.filtered().covariance());
-
-            // Eq (3.38) of "Pattern Recognition, Tracking and Vertex
-            // Reconstruction in Particle Detectors"
-            const matrix_type<e_bound_size, e_bound_size> smoothed_cov_inv =
-                predicted_cov_inv + filtered_cov_inv;
-
-            const matrix_type<e_bound_size, e_bound_size> smoothed_cov =
-                matrix_operator().inverse(smoothed_cov_inv);
-
-            // Eq (3.38) of "Pattern Recognition, Tracking and Vertex
-            // Reconstruction in Particle Detectors"
-            const matrix_type<e_bound_size, 1u> smoothed_vec =
-                smoothed_cov *
-                (filtered_cov_inv * trk_state.filtered().vector() +
-                 predicted_cov_inv * predicted_vec);
-
-            trk_state.smoothed().set_vector(smoothed_vec);
-            trk_state.smoothed().set_covariance(smoothed_cov);
-
-            const matrix_type<D, 1> residual_smt =
-                meas_local - H * smoothed_vec;
-
-            // Eq (3.39) of "Pattern Recognition, Tracking and Vertex
-            // Reconstruction in Particle Detectors"
-            const matrix_type<D, D> R_smt =
-                V - H * smoothed_cov * matrix_operator().transpose(H);
-
-            // Eq (3.40) of "Pattern Recognition, Tracking and Vertex
-            // Reconstruction in Particle Detectors"
-            const matrix_type<1, 1> chi2_smt =
-                matrix_operator().transpose(residual_smt) *
-                matrix_operator().inverse(R_smt) * residual_smt;
-
-            trk_state.smoothed_chi2() =
-                matrix_operator().element(chi2_smt, 0, 0);
-        }
+        // Wrap the phi in the range of [-pi, pi]
+        wrap_phi(trk_state.filtered());
 
         return true;
     }
diff --git a/core/include/traccc/fitting/kalman_filter/kalman_actor.hpp b/core/include/traccc/fitting/kalman_filter/kalman_actor.hpp
index aae38a507..fd99cfa52 100644
--- a/core/include/traccc/fitting/kalman_filter/kalman_actor.hpp
+++ b/core/include/traccc/fitting/kalman_filter/kalman_actor.hpp
@@ -140,8 +140,7 @@ struct kalman_actor : detray::actor {
 
             const bool res =
                 sf.template visit_mask<gain_matrix_updater<algebra_t>>(
-                    trk_state, propagation._stepping.bound_params(),
-                    actor_state.backward_mode);
+                    trk_state, propagation._stepping.bound_params());
 
             // Abort if the Kalman update fails
             if (!res) {
diff --git a/core/include/traccc/fitting/kalman_filter/kalman_fitter.hpp b/core/include/traccc/fitting/kalman_filter/kalman_fitter.hpp
index b89bdfc01..d8c17d408 100644
--- a/core/include/traccc/fitting/kalman_filter/kalman_fitter.hpp
+++ b/core/include/traccc/fitting/kalman_filter/kalman_fitter.hpp
@@ -12,11 +12,14 @@
 #include "traccc/edm/track_candidate.hpp"
 #include "traccc/edm/track_parameters.hpp"
 #include "traccc/edm/track_state.hpp"
+#include "traccc/finding/actors/interaction_register.hpp"
+#include "traccc/fitting/actors/surface_id_aborter.hpp"
 #include "traccc/fitting/fitting_config.hpp"
 #include "traccc/fitting/kalman_filter/gain_matrix_smoother.hpp"
 #include "traccc/fitting/kalman_filter/kalman_actor.hpp"
 #include "traccc/fitting/kalman_filter/kalman_step_aborter.hpp"
 #include "traccc/fitting/kalman_filter/statistics_updater.hpp"
+#include "traccc/fitting/kalman_filter/two_filters_smoother.hpp"
 #include "traccc/utils/particle.hpp"
 
 // detray include(s).
@@ -63,11 +66,11 @@ class kalman_fitter {
     using fit_actor = traccc::kalman_actor<algebra_type, vector_type>;
     using resetter = detray::parameter_resetter<algebra_type>;
 
+    // Forward Propagator type
     using actor_chain_type =
         detray::actor_chain<detray::dtuple, aborter, transporter, interactor,
                             fit_actor, resetter, kalman_step_aborter>;
 
-    // Propagator type
     using propagator_type =
         detray::propagator<stepper_t, navigator_t, actor_chain_type>;
 
@@ -207,13 +210,78 @@ class kalman_fitter {
     /// track and vertex fitting", R.Frühwirth, NIM A.
     ///
     /// @param fitter_state the state of kalman fitter
-    TRACCC_HOST_DEVICE
-    void smooth(state& fitter_state, const bound_covariance& /*cov*/) {
+    TRACCC_HOST_DEVICE void smooth(state& fitter_state,
+                                   const bound_covariance& /*cov*/) {
+
+        auto& track_states = fitter_state.m_fit_actor_state.m_track_states;
+
+        // Since the smoothed track parameter of the last surface can be
+        // considered to be the filtered one, we can reversly iterate the
+        // algorithm to obtain the smoothed parameter of other surfaces
+        auto& last = track_states.back();
+        last.smoothed().set_parameter_vector(last.filtered());
+        last.smoothed().set_covariance(last.filtered().covariance());
+        last.smoothed_chi2() = last.filtered_chi2();
 
         if (m_cfg.use_backward_filter) {
+            /*
             // Create propagator
-            propagator_type propagator(m_cfg.propagation);
+            backward_propagator_type propagator(m_cfg.propagation);
+
+            for (typename vector_type<
+                     track_state<algebra_type>>::reverse_iterator it =
+                     track_states.rbegin();
+                 it != track_states.rend() - 1; ++it) {
+
+                // Get surface corresponding to bound params
+                const detray::tracking_surface sf{m_detector,
+                                                  it->surface_link()};
 
+                const typename detector_type::geometry_context ctx{};
+
+                auto bound_params = it->smoothed();
+                // inflate_covariance(bound_params, m_cfg.)
+
+                // Apply material interaction
+                typename interactor::state interactor_state;
+                interactor{}.update(
+                    ctx,
+                    detail::correct_particle_hypothesis(m_cfg.ptc_hypothesis,
+                                                        bound_params),
+                    bound_params, interactor_state,
+                    static_cast<int>(detray::navigation::direction::e_backward),
+                    sf);
+
+                // Two filters (forward & backward) method
+                typename backward_propagator_type::state propagation(
+                    bound_params, m_field, m_detector);
+                propagation._navigation.set_volume(it->surface_link().volume());
+
+                propagation._navigation.set_direction(
+                    detray::navigation::direction::e_backward);
+
+                typename aborter::state path_abrt_state;
+                typename transporter::state trp_state;
+                surface_id_aborter::state id_abrt_state{
+                    (it + 1)->surface_link()};
+                typename interactor::state int_state;
+                typename interaction_register<interactor>::state reg_state{
+                    int_state};
+                typename resetter::state rst_state;
+
+                propagator.propagate(
+                    propagation,
+                    detray::tie(path_abrt_state, trp_state, id_abrt_state,
+                                reg_state, int_state, rst_state));
+
+                const detray::tracking_surface sf_next{
+                    m_detector, (it + 1)->surface_link()};
+
+                sf_next.template visit_mask<two_filters_smoother<algebra_type>>(
+                    *(it + 1), propagation._stepping.bound_params());
+            }
+            */
+            
             // Set path limit
             fitter_state.m_aborter_state.set_path_limit(
                 m_cfg.propagation.stepping.path_limit);
@@ -221,9 +289,6 @@ class kalman_fitter {
             // Seed param for backward seed = last state of forward filter
             auto bw_seed_params =
                 fitter_state.m_fit_actor_state.m_track_states.back().filtered();
-            inflate_covariance(bw_seed_params,
-                               m_cfg.covariance_inflation_factor);
-            //bw_seed_params.set_covariance(cov);
 
             // Two filters (forward & backward) method
             typename propagator_type::state propagation(bw_seed_params, m_field,
@@ -239,7 +304,9 @@ class kalman_fitter {
 
             // Reset the backward mode to false
             fitter_state.m_fit_actor_state.backward_mode = false;
+
         } else {
+            /*
             auto& track_states = fitter_state.m_fit_actor_state.m_track_states;
 
             // The Rauch-Tung-Striebel(RTS) smoother requires the following:
@@ -253,6 +320,7 @@ class kalman_fitter {
             last.smoothed().set_parameter_vector(last.filtered());
             last.smoothed().set_covariance(last.filtered().covariance());
             last.smoothed_chi2() = last.filtered_chi2();
+            */
 
             for (typename vector_type<
                      track_state<algebra_type>>::reverse_iterator it =
diff --git a/core/include/traccc/fitting/kalman_filter/two_filters_smoother.hpp b/core/include/traccc/fitting/kalman_filter/two_filters_smoother.hpp
new file mode 100644
index 000000000..930901114
--- /dev/null
+++ b/core/include/traccc/fitting/kalman_filter/two_filters_smoother.hpp
@@ -0,0 +1,124 @@
+/** TRACCC library, part of the ACTS project (R&D line)
+ *
+ * (c) 2024 CERN for the benefit of the ACTS project
+ *
+ * Mozilla Public License Version 2.0
+ */
+
+#pragma once
+
+// Project include(s).
+#include "traccc/definitions/qualifiers.hpp"
+#include "traccc/definitions/track_parametrization.hpp"
+#include "traccc/edm/track_state.hpp"
+
+namespace traccc {
+
+/// Type unrolling functor for two-filters smoother
+template <typename algebra_t>
+struct two_filters_smoother {
+
+    // Type declarations
+    using matrix_operator = detray::dmatrix_operator<algebra_t>;
+    using size_type = detray::dsize_type<algebra_t>;
+    template <size_type ROWS, size_type COLS>
+    using matrix_type = detray::dmatrix<algebra_t, ROWS, COLS>;
+
+    /// Two-filters smoother operation
+    ///
+    /// @param mask_group mask group that contains the mask of surface
+    /// @param index mask index of surface
+    /// @param trk_state track state of the surface
+    /// @param bound_params bound parameter
+    ///
+    /// @return true if the update succeeds
+    template <typename mask_group_t, typename index_t>
+    TRACCC_HOST_DEVICE inline bool operator()(
+        const mask_group_t& /*mask_group*/, const index_t& /*index*/,
+        track_state<algebra_t>& trk_state,
+        const bound_track_parameters& bound_params) const {
+
+        using shape_type = typename mask_group_t::value_type::shape;
+
+        const auto D = trk_state.get_measurement().meas_dim;
+        assert(D == 1u || D == 2u);
+        if (D == 1u) {
+            return update<1u, shape_type>(trk_state, bound_params);
+        } else if (D == 2u) {
+            return update<2u, shape_type>(trk_state, bound_params);
+        }
+
+        return false;
+    }
+
+    template <size_type D, typename shape_t>
+    TRACCC_HOST_DEVICE inline bool update(
+        track_state<algebra_t>& trk_state,
+        const bound_track_parameters& bound_params) const {
+
+        assert(trk_state.filtered().surface_link() ==
+               bound_params.surface_link());
+
+        static_assert(((D == 1u) || (D == 2u)),
+                      "The measurement dimension should be 1 or 2");
+
+        const auto meas = trk_state.get_measurement();
+
+        matrix_type<D, e_bound_size> H = meas.subs.template projector<D>();
+
+        // Measurement data on surface
+        const matrix_type<D, 1>& meas_local =
+            trk_state.template measurement_local<D>();
+
+        // Predicted vector of bound track parameters
+        const matrix_type<e_bound_size, 1>& predicted_vec =
+            bound_params.vector();
+
+        // Predicted covaraince of bound track parameters
+        const matrix_type<e_bound_size, e_bound_size>& predicted_cov =
+            bound_params.covariance();
+
+        const matrix_type<e_bound_size, e_bound_size> predicted_cov_inv =
+            matrix_operator().inverse(predicted_cov);
+        const matrix_type<e_bound_size, e_bound_size> filtered_cov_inv =
+            matrix_operator().inverse(trk_state.filtered().covariance());
+
+        // Eq (5.1-11) of "Applied Optimal Estimation"
+        const matrix_type<e_bound_size, e_bound_size> smoothed_cov_inv =
+            predicted_cov_inv + filtered_cov_inv;
+
+        const matrix_type<e_bound_size, e_bound_size> smoothed_cov =
+            matrix_operator().inverse(smoothed_cov_inv);
+
+        // Eq (5.1-12) of "Applied Optimal Estimation"
+        const matrix_type<e_bound_size, 1u> smoothed_vec =
+            smoothed_cov * (filtered_cov_inv * trk_state.filtered().vector() +
+                            predicted_cov_inv * predicted_vec);
+
+        trk_state.smoothed().set_vector(smoothed_vec);
+        trk_state.smoothed().set_covariance(smoothed_cov);
+
+        const matrix_type<D, 1> residual_smt = meas_local - H * smoothed_vec;
+
+        // Spatial resolution (Measurement covariance)
+        const matrix_type<D, D> V =
+            trk_state.template measurement_covariance<D>();
+
+        // Eq (3.39) of "Pattern Recognition, Tracking and Vertex
+        // Reconstruction in Particle Detectors"
+        const matrix_type<D, D> R_smt =
+            V - H * smoothed_cov * matrix_operator().transpose(H);
+
+        // Eq (3.40) of "Pattern Recognition, Tracking and Vertex
+        // Reconstruction in Particle Detectors"
+        const matrix_type<1, 1> chi2_smt =
+            matrix_operator().transpose(residual_smt) *
+            matrix_operator().inverse(R_smt) * residual_smt;
+
+        trk_state.smoothed_chi2() = matrix_operator().element(chi2_smt, 0, 0);
+
+        return true;
+    }
+};
+
+}  // namespace traccc