I know that the Bachmann Plymouth is based somewhat on fantasy, but what years would be appropriate for something like this to be running?
Over 7,500 Plymouth locomotives were constructed in Plymouth, Ohio of which at least 500 are still in active use. With that said, I would model from post WWII to the late 1970s if it were me. The Plymouth Locomotive Works built its last locomotive in 1997. The rights to the spare parts were then sold to the Williams Distribution Co. Most Plymouth locomotive were 25 tons or less.
That is sounding a lot like the 50's to me. Thank you for your feedback. I appreciate that.