Some European transit systems have cars with track brakes. These brakes, used for emergencies only, have pads that go against the rails. That's one way to avoid flat wheels and to be able to stop quickly, even on grades.

Don't know offhand if there is any U.S. equipment with these

There was even one case where track brakes were used on mainline diesels. The Austrian Erzberg (ore mountain) line was originally a cog rail operation. It had/has grades in the 7 per cent range. After the cog rail wore out, it was determined that the ore deposits in the area would only last a couple more years -- and rebuilding the the cog rail to heavier standards -- as well as ordering new rack engines was not economical. So, for the last couple of years, the ore trains operated (loads downhill, empties uphill) adhesion only, with 10 loaded ore cars and two 1,500 hp diesels. (These diesels had track brakes installed -- in addition to their normal dynamic and air brakes.)

The track brakes were tied to an automatic overspeed sensor. When the system was armed, excessive speed would automatically trigger an emergency application, including use of the track brakes. The overspeed sensor could be disabled to allow the locomotives to operate at higher speeds on normal lines in flat terrain.

I rode this line in the 1970s, on an adhesion-only riilbus, back when it was still a rack opertion, using two steam rack locomotives on each ore train. The railbus also had track brakes with an overspeed sensor. A large red metal flag in the cab was raised when the track brake system was armed.

The line still exists today -- as a museum railroad, with tourists carried on these preserved adhesion-only railbuses.

Probably more than you wanted to know.