Increase Train car/station holding capacity/throughput by 100% base, with research for later game.
Having advanced into early late game, I can say that trains are awesome. However, trains are not practical for use in a lot of cases for me. It's awesome having trains move tons of materials around, but there seems to be a hard tradeoff in using trains vs trucks at lengths over of travel distance. The ratio of items hauled to train length is just not worth it to clog up lines or have 40 unit consists, especially with maps with elevation. I would like to see a base 100% increase to train car capacity and station building storage. This would make them MUCH more attractive to use for very long distances.
I would also like to see some late game infinite research to increase hauling capacity. The above increase in carrying capacity can be also achieved through research, maybe using the alloy mixer to allow for Steel/Aluminum alloy cars that can haul more but the increased cost is the trade off (better materials that weigh less means you can haul more for a given weight, kinda thing.)