If you don't mind using mods check out Burny's Train Station Blueprint Creator. It can generate any custom train loader/unloader you might ever need.
Very fast train unloader very heavily based on this blueprint by Jan Streffing.
If you want the belts to go in the other direction use this blueprint.
Changes made to the design:
- Exported string for Factorio 0.15
- Swapped stack filter inserters with stack inserters
- Changed circuit logic to work with all input items
- Removed coal line to locomotive. Feel free to add your own or use requester chests.