Well, there are a couple things wrong there. First, there are no steam trains in Cities: Skylines. But there are a lot of custom train models in the game’s Steam Workshop. But whether there are any steam trains in the game, I have come to the conclusion that metros are boring.
My main objection to using metros in Cities: Skylines, is not that the system sucks or anything along those lines. It is solely because you cannot see what is going on, unless you open up the underground view. Sure, metros do save space on the ground and allows you to have very neat above ground designs, hiding things from view, but I like the mess of a train network snaking its way through suburbs. Even then, the metro vehicles are literally a series of boxes. If you do not believe me, download the Advanced Vehicle Operations mod and see for yourself.
The train systems in Cities: Skylines, on the other hand, have far more support in the Workshop. You can build some crazy complex systems and the ability to choose rolling stock on lines is a nice little perk. My personal favourite is building a central station using the Modular Old Brick Central components and then spreading a network out from the CBD of a city. There are other options for central stations in the Workshop, so have a look for yourself.