When my son was little, we used painters tape to make roads on the floor of our playroom. You can draw lines down the center with Sharpie and use your imagination.
Apparently enough parents have done this, that some clever toy industry folks decided to turn the DIY approach into a product.
There are train tracks, which seems handy for travel if you want to just bring 3-4 trains and this roll of tape for your train addict, but I’m not sure any hardcore track lover would be satisfied with this substitute. What do you think?
find railroad tape on Amazon
See also, Civil Engineer tape.