Thanks again for everyone’s input and help.

from this post it sounds like you have verified that the transformer is fine and the connection to the track is fine (and that you worked out where to place the leads on the track to check the voltage). Which leaves the engine as the problem

Have you made sure that the wheels on the tender and cars run freely?

One common problem is bad electrical contact somewhere in the circuit.

Here are some basic checks for ensuring good electrical contact, this may help:

1) Clean the rail tops: paper towel is good, wetted with meths or lighter fluid (warning: no smoking !) if there is grease or oil. Personally, I disagree with track rubbers, they just leave bits of grit and rubber on the track, causing further problems. Modern nickel-silver track does not suffer corrosion indoors.

2) Clean the wheel rims: a wire brush is good, a rotary wire brush on a dremel type tool is brilliant. Croc clip leads from controller to the motor to turn the wheels makes this process very easy.

3) Clean the inner faces of wheels (or the axles) and the pickup contacts touching them.

4) Old motors can benefit from cleaning of the brushes and commutator.

5) Some basic controllers are rather feeble in their output, and will repeatedly cut-out if running a loco that takes a high current. To check this, fit a little 12V bulb on your controller output to see if it is still supplying power when the loco unexpectedly stops. Old locos often take more current.

6) Rail joiners are not that brilliant at electrically connecting rails. Have the controller wire connected to the track at several places (depending on layout size). (Observe consistent polarity !)
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Another possibility with second hand locos is mechanical friction. If you have the ability and tools, carefully disassemble the gear trains, clean and lubricate everything and make sure nothing is jamming.