Have a look at Auto Lift Parts - Automotive Lifts Repair Parts - Car Lift Repair for parts and what is under the floor.

Older lifts didn't have anti rotation pins for a reason, it was to allow the operator to rotate the car before exiting the shop ( Rotary Lift was a early example )

With some digging you might be able to find a 4 arm head, this would be much more usefull. As for rust, does the area have a high water table or is it very dry? The old rail head has value in the beams so no loss there.

Are you looking for the old time feel or just a place to work on your car? If the latter, buy a above ground lift. These are no big deal to install and a good one isn't too expensive ( $ 3,000 + ) Even a used on can be a good deal if you can tune it up.