We've been finding that trains so fundamentally change things, for the better, that we wish they had been available earlier in the game when base layout was still in infant stages.
One thought, and I promised I'd pass it on, would be to move trains to tier 4 using a fueled locomotive, and then the electric locomotive could come in tier 6 as it is now. This would, of course, require changes to the recipes for the train station buildings and the freight yard as well as the addition of a fuel inlet for the train station. A Mk2 Freight Yard could be an option, which is the current one, whereas a new Mk1 would have only one inlet/outlet and a smaller capacity (maybe 24 or 32). Fuel reserves in the locomotive would need to be large enough to be usable as you might even be legitimately running the train on coal.
One balance possibility would be to limit the number of cars in a fueled train to, say, three or four, and not allow the train to go backwards, requiring at minimum a loop to turn it around.