Before you lock yourself in, let's examine more possibilities. Is there any possibility of a "shelf" type layout against the walls? Put it up fairly high, say, 50" or even higher, and use the space underneath just like you are using it now. With a shelf, there is no need for legs, and you may get a much longer mainline run. The shelves don't have to be much wider than 12 inches, and could go as big as 24" deep.
Now, one thing to consider is your wants in running trains. Are you content to sit back and watch them go in circles with continuous running, or do you want more prototypical operations with picking up and setting out cars at industries? A complete loop is not an absolute must. "Point-to-point" operations can be just as satisfying, if not more than, a continuous loop.