And if you get CAUGHT allowing the engines to slip, thereby damaging the rail(s), you're gonna get some "whammy" time! (at least on NS) Plus you're gonna get a cussin' from the Track Supervisior if he sees you doing that. Of course, ALL engines slip at times, but its continuing to allow such that causes rail "cupping".
Personally, I never understood this fascination that rail buffs seem to have with slipping wheels. It's NOT funny, but occurred a LOT on our shop tracks where heavy machinery such as Tampers, and Ballast Regulators, and Cranes sometimes became stalled due to drive train failures and could not proceed. This required us---even using fork lifts---to come and shove them ahead into a stall or onto the transfer table.
One time, to show us how mechanical engineers sometimes make boo-boos, They got the bright idea to move the controls of a walking machine so they would be better accessed and the operator, walking along beside the RoW, didn't have to stretch to work the controls. All the heavy components were over the drive axle and provided proper wheel loading and traction. Not taking this into consideration, the engineer-in-charge moved the controls, and several other heavy components to the OTHER end of the machine, effectively removing all loading from the drive axle (it only used the one axle for propulsion). I came around the shop on a fork lift to find this machine just outside a work bay, its diesel engine roaring, hopelessly slipping its wheels and going NOWHERE! Clearly this wasn't going to work, and after shoving the machine into the back shop, the engineers and the workers set about correcting this faux pas!