Easiest maps are 2-6 and 3-6
Just play normally until end of game, but keep 1 last train on tracks, no problem if it worths 0$.
If you played nicely, you must have a huge amount of $ (between 200 and 300K).
Now, the best thing to do is to fill the map with tracks, as much as you can! But try to avoid bulldozing expensive buildings (You don't really have to FILL every square of the whole map).
Achievement will pop after you ended the map (when your last train entered his station).