Plarail Catalog Feb-88, with commentary

... I thought the design of Plarail contacts made it virtually impossible to actually fit the battery in reverse? I tried it once just to see if I could get an engine running in reverse for a shunting loop ... but alas, no, because the positive contact [on the motor unit] was recessed to prevent such an occurrence, by making sure you had to put the raised 'tip' on the positive end onto the correct contact to fit.

With the 'Old Power' style metal-cased motor/gearbox it is quite easy to insert the battery the wrong way around and run the motor in reverse. I just tried it in a front-switch D51.