link to the series: https://www.youtube.com/watch?v=XFB8rn2 ... ObizXMXBPo.
The requirements are:
- must be exact
- you can change the items you want loaded into the train on a combinator
- can remove garbage items/ if some item's amount is to high
- can handle multiple wagons
Fooluaintblack's best design:

features:
- combinators: 17 decider combinators, 7+2l arithmetic combinator, and 3+l constant combinators where l is the number of wagons
- height: 12 blocks
- speed: 834 ticks
- test setup blueprint:

features:
- combinators: 17+32l decider combinators, 7+25l arithmetic combinator, and 2+l constant combinators where l is the number of wagons
- height: 26 blocks
- speed: 777 ticks
- test setup blueprint:
If anyone is interested then I will try to explain how it works.
I also want to challenge you, the reader, to come up with a faster design.
ps: when you open the blueprint, send the train first to the other station.
ps: English isn't my native langue so sorry for grammar/spelling mistakes.
edit:
I managed to improve my new design even more
Currently my best design:

features:
- combinators: 16+37l decider combinators, 8+18l arithmetic combinator, and 4+l constant combinators where l is the number of wagons
- height: 24 blocks
- speed: 658 ticks
- test setup blueprint: