1. A small railroad engine is able to exert 8•105 N of pulling force. The coefficient of friction for steel wheels on steel rail is 0.4. Each rail car has the approximate mass of 20 tons
How many cars can this engine pull on a level track?
This is a ridiculous question. When wheels roll, there is negligible friction. No one would try to pull a train with locked wheels, sliding on the track.
That is why the wheel is such a great invention.