Depends on if you're aiming for a certain area or general Maine. I have a couple of New England books, a general map shows
BAR on a North-South route between Searsport and Fort Kent.
CP had a line across the middle.
MEC had tracks in southern Maine.
GT came in by way of Gorham, NH and went to Portland. Since GT shared power with CV, you could mix CV-GT units. Also both were once involved with CN so you could have older CN power.
BM came from Boston to Portland.
Sometimes BM and CV had power pools, same with BM & MEC.
Today, MMA has the former BAR and CP lines.
I believe Guilford has the BM & MEC tracks.
I don't know who if anyone has the GT line.
A good site to bookmark is this
http://trains.rockycrater.org/pfmsig/index.php
They have state by state maps listed at the link
"1948 Atlas" showing the lines.