Building the loco plants first will make the stock cheaper, this is true, but it will also mean we continue to have no trains while trying to industrialize which is disastrous. I don't think we need to build spur lines off into the Siberian wastes for cities that won't be built for years to contain intellectuals that don't exist yet, when the current rail lines connecting all our current infrastructure are already starved for stock. Uralmash can make all the lathes in the world, the Donbas can dig up all the coal in the world, Baku can refine all the fuel in the world, and every gram of it is utterly useless if we can't get it where it's going. I stand by my choice to build trains now, the locomotive plants probably won't pay for themselves until years in the future (possibly not even in this Plan) and we need a shot in the arm for our logistical network now.
One of the major historical challenges of the real 5 Year Plans was not having enough rolling stock to move things where they were needed, people starved while grain sat in warehouses 2000km away unable to be moved to them and all that. We don't even have to import the full locomotives, we can build them mostly domestic and just need some foreign finishing touches, as an emergency measure to unparalyze our logistical network I think it's worth the investment this first round. Later we can build the plants and continue to expand our domestic train production because much like coal there's no such thing as too many trains, but I'm convinced we need this emergency shot now.
Also I expect just about every major new capital project will be improved or tweaked somehow by the Industrial Council, I don't want to break ground on locomotive plants that they turn around and declare obsolete/inefficient/horribly designed as soon as the nerds settle in. Building old coal locomotives in the already existing plants will definitely be mildly obsolete but they will work and they will match the rest of the existing stock and infrastructure, providing a solid foundation to build a more modern rail system off of.