TL;DR
A chest type should be 2x1 instead of currently all chests being 1x1 to allow train-to-train inserter tranfers. Wood chests seem like the best candidate for this.What ?
Because train tracks are on a 2x2 grid it's impossible to transfer items directly between two non-diagonal trains using regular inserters. Currently it can only be done with long-handed inserters, belts, or bots. It's odd that regular sized inserters are excluded from this list.All chests in the game, wood, iron, steel, and all logistics chests are 1x1. If one of these chest types were instead 2x1 it would enable transferring items from one train to another.
Why ?
Almost everything in Factorio can be implemented modularly. However if there are two train networks, it's currently not possible to transfer items with regular inserters and chests between the different trains. The inserter is the standard way of moving stacks of items in Factorio and should be able to handle this problem. Part of the beauty of Factorio is that there are many ways to solve the same problem, forcing the use of belts, bots, or long-handed inserters goes against this.Wood chests seem like the best candidate for a 2x1 chest because it would make early game layouts transition into mid game layouts more interesting. Right now there is very little incentive to use the iron chest, it's quite common to just use wood chests until steel chests are available. If wood chests were instead 2x1 it could encourage a faster transition to iron chests as a secondary benefit to allowing train-to-train transfer with inserters and chests. Additionally, wood chests being cruder and having a larger footprint fits thematically with them being low-tech chests.