I did one of these a few years ago. You can mount the mechanism either on top or below the table. I opted to mount as much as possible below the table to increase the illusion that it is self propelled. 3/8" rod is OK but the thicker the rod the stiffer it will be. A slight bend will cause a wobble. I had a few DC gearmotors I salvaged off old instruments. Voltage rating is flexible. The motor I used is rated for 36 volts but I am using 16 VDC. Speed is not an issue. You can find them for under 20 bucks on eBay. There are limit switches on both ends. They are SPDT with a diode on one side so after the switch is activated it can only run in the opposite direction. A DPDT switch is used to change direction. I tapped a piece of aluminum square stock and attached a piece of spring tempered copper to it. The copper sheet has a hole in it that clips onto one of the wheel axles that has been replaced with a longer one though a slot in the table. You could also operate it with a TMCC controller but as mine only gets used infrequently on our modular layout I haven't implemented that option. The Crane is TMCC but I found response is very slow compared with using the switch controller supplied with the crane that I don't use that feature either. Its a big hit with the kids both young and old.
Here are some pics.
Pete
