There are some missing requirement for producing the locomotive, I don't know if those apply to vanilla game, since I haven't played it in years.
Currently, it only requires Chemical tech card, but to create a locomotive, it requires Big electric motor and Rocket control unit.
The technology for those two items are unlocked after Chemical tech card technology is unlocked, so even though, theoretically you could build one, you can't craft the needed material yet.
And a suggestion for last, you could use low density structure instead of steel plates to explain the low weight of the locomotive.
If you decide to increase the technology requirement as discussed on the other thread, you could even swap advanced circuit for process unit to make more sense.

I have only added functional compatibility to SE. This mod was intended to be used with vanilla and none of these issues occur. I spent some time making this mod work with SE so a wider number of players can use it. Whether the tech trees make total sense in these edge cases is not something I will look into as I'm not going to try and fit around many other mods.

Part of the SE compatibility was changing processing units to advanced circuits to make the mod a bit more attainable in my eyes. The vanilla game uses processing units.

Using LDS for the trains would've made sense, but I don't want to change the recipes unless releasing a major update and it seems justified enough.

I develop and use this mod without any overhauls installed, and that's just how I'm going to keep developing it for now :)

I've since started an SE play through with ET 0.2.X and will now update the tech tech prerequisites so they make sense in SE :)