diff --git a/cuslines/cuda_python/cu_propagate_seeds.py b/cuslines/cuda_python/cu_propagate_seeds.py
index c1d8a8b..cbe4fb1 100644
--- a/cuslines/cuda_python/cu_propagate_seeds.py
+++ b/cuslines/cuda_python/cu_propagate_seeds.py
@@ -256,5 +256,40 @@ def _yield_slines():
 
         return _yield_slines()
 
+    def gen_bin_indices(self, bin_starts, bin_len):
+        bin_edges = np.append(bin_starts, bin_starts[-1] + bin_len)
+        bin_indices = {k: [] for k in range(len(bin_starts))}
+        
+        for ii in range(self.ngpus):
+            scaled_lens = self.sline_lens[ii] * self.gpu_tracker.step_size
+            assignments = np.digitize(scaled_lens, bin_edges) - 1
+            
+            for k in range(len(bin_starts)):
+                jj_indices = np.where(assignments == k)[0]
+                if jj_indices.size > 0:
+                    bin_indices[k].append((ii, jj_indices))
+
+        return bin_indices
+
+    def as_array_sequence_group(self, bin_indices, bin_start):
+        relevant_blocks = bin_indices[bin_start]
+
+        def _yield_slines():
+            for ii, jj_array in relevant_blocks:
+                gpu_slines = self.slines[ii]
+                gpu_lens = self.sline_lens[ii]
+                for jj in jj_array:
+                    npts = gpu_lens[jj]
+                    yield np.asarray(gpu_slines[jj], dtype=REAL_DTYPE)[:npts]
+
+        def _get_buffer_size():
+            total_pts = 0
+            for ii, jj_array in relevant_blocks:
+                total_pts += np.sum(self.sline_lens[ii][jj_array])
+
+            return math.ceil((total_pts * 3 * REAL_SIZE) / MEGABYTE)
+
+        return ArraySequence(_yield_slines(), _get_buffer_size())
+
     def as_array_sequence(self):
         return ArraySequence(self.as_generator(), self.get_buffer_size())
diff --git a/cuslines/cuda_python/cu_tractography.py b/cuslines/cuda_python/cu_tractography.py
index 7064cc9..466a326 100644
--- a/cuslines/cuda_python/cu_tractography.py
+++ b/cuslines/cuda_python/cu_tractography.py
@@ -313,3 +313,74 @@ def generate_trx(self, seeds, ref_img):
         trx_file.resize()
 
         return trx_file
+
+    def generate_trx_grouped_by_len(self, seeds, ref_img, min_len=20, max_len=250, bin_len=10):
+        global_chunk_sz, nchunks = self._divide_chunks(seeds)
+
+        # Will resize by a factor of 2 if these are exceeded
+        sl_per_seed_guess = 3
+        n_sls_guess = sl_per_seed_guess * seeds.shape[0]
+
+
+        bin_starts = np.arange(min_len, max_len + bin_len, bin_len)
+        trx_files = {}
+        offsets_idxs = {}
+        sls_data_idxs = {}
+        for bin_start in bin_starts:
+            max_steps = (bin_start + bin_len) / self.step_size
+            trx_files[bin_start] = TrxFile(
+                reference=ref_img,
+                nb_streamlines=n_sls_guess,
+                nb_vertices=n_sls_guess * max_steps,
+            )
+            trx_files[bin_start].streamlines._offsets = \
+                trx_files[bin_start].streamlines._offsets.astype(np.uint64)
+            offsets_idxs[bin_start] = 0
+            sls_data_idxs[bin_start] = 0
+
+        with tqdm(total=seeds.shape[0]) as pbar:
+            for idx in range(int(nchunks)):
+                self.seed_propagator.propagate(
+                    seeds[idx * global_chunk_sz : (idx + 1) * global_chunk_sz]
+                )
+                bin_indices = self.seed_propagator.gen_bin_indices(bin_starts, bin_len)
+                for bin_start in bin_starts:
+                    tractogram = Tractogram(
+                        self.seed_propagator.as_array_sequence_group(bin_indices, bin_start),
+                        affine_to_rasmm=ref_img.affine,
+                    )
+                    tractogram.to_world()
+                    sls = tractogram.streamlines
+
+                    new_offsets_idx = offsets_idxs[bin_start] + len(sls._offsets)
+                    new_sls_data_idx = sls_data_idxs[bin_start] + len(sls._data)
+
+                    if (
+                        new_offsets_idx > trx_files[bin_start].header["NB_STREAMLINES"]
+                        or new_sls_data_idx > trx_files[bin_start].header["NB_VERTICES"]
+                    ):
+                        logger.info("TRX resizing...")
+                        trx_files[bin_start].resize(
+                            nb_streamlines=new_offsets_idx * 2,
+                            nb_vertices=new_sls_data_idx * 2,
+                        )
+
+                    # TRX uses memmaps here
+                    trx_files[bin_start].streamlines._data[sls_data_idxs[bin_start]:new_sls_data_idx] = sls._data
+                    trx_files[bin_start].streamlines._offsets[offsets_idxs[bin_start]:new_offsets_idx] = (
+                        sls_data_idxs[bin_start] + sls._offsets
+                    )
+                    trx_files[bin_start].streamlines._lengths[offsets_idxs[bin_start]:new_offsets_idx] = (
+                        sls._lengths
+                    )
+
+                    offsets_idxs[bin_start] = new_offsets_idx
+                    sls_data_idxs[bin_start] = new_sls_data_idx
+                pbar.update(
+                    seeds[idx * global_chunk_sz : (idx + 1) * global_chunk_sz].shape[0]
+                )
+
+        for bin_start in bin_starts:
+            trx_files[bin_start].resize()
+
+        return trx_files