In old books you can see lash-ups using car batteries and rheostats. In this day and age, you are going to have a far easier time finding a secondhand Lionel type 4044 than you will finding a rheostat. Plus, a proper train transformer is far safer for you and your trains. It's not worth getting a shock or burning your house down to save a buck. Whoever you sell your trains to won't mind buying an extra transformer with them.