Cant build steam engine :/

[MeGa]

IT says I must demolish the previous engine, and I dont really know how or exactly what is it, as I cant dismount the trolley thing to make wagon faster, which should be the engine?

OR maybe its history gated? I got all the materials though

[Quill]

So, in order to build a steam engine, you need your front wagon to be at least 3 tiles wide. (1 tile being a 2x2 area of wooden planks). You then need to remove the trolley lever and the horn at the next depo you arrive at. (You only need to remove the horn to make space. But you must remove the trolley lever to be able to build the engine itself.)