Steam locomotive driving wheels require heavy counterweights to balance the side rods and pistons. Because these reciprocating parts move horizontally, the rotating counterweights create a severe vertical imbalance. At high speeds, this imbalance slams the wheel down into the rail once per revolution, causing immense stress to both the track structure and the wheel bearings.
Heard on the job
We had to lower the track speed limit on that branch line because the hammer blow from the heavy freight engines was destroying the rail joints.
Insider tip
To minimize this track damage, mechanical engineers historically had to compromise between balancing the locomotive for a smooth ride and protecting the rail from excessive vertical pounding.