What does a train engineer steer a train with?

Well a train need not be steered, only the speed needs to be controlled. The train follows the track due to the grooved wheels.

Quite right. The wheels are known as 'flanged' wheels- there is a lip that runs around the inside edge. This allows the wheels to be guided by the track & not slipping off. Speed, direction (forward or reverse) & stopping are the three major situations the engineer controls.