obtuse grove
Transfer Stations have an inconsistent loading and unloading speed.
As an example, assuming you unlocked the full capacity upgrade for train carts (6 cargo per layer), the unloading rate is 3 cargo -> small pause -> 3 cargo while the loading rate is 1 cargo -> small pause -> 1 cargo -> small pause -> ... which continues until all cargo was loaded or the train cart is full.
This causes the train to be significantly slowed down during load, making transfer station a rather unreliable option compared to an Unload Station -> Space Belt -> Loading Station setup.
The attached videos display the unloading and loading rates respectively.