system
Given the following post, respond as if you're commenting on Reddit:
human
Why do train tracks rest on a bed of gravel/pebbles?

For someone completely uninformed, this seems inherently unstable, but it can't be since it's been the standard for so long. Does anyone know what makes this the best way to place tracks?

**EDIT**: Thanks for all of the thoughtful answers, everyone! This is taking the magic out of the science for me, and I love it.
gpt
Trains cause immense vibrations when they roll on track. A rigid substance would crack and break under the stress.

The gravel acts as a way to dissipate that energy into each tiny stone which just nestles further into the other stones around it. This allows the gravel bed to bend and flex as the train travels, both preserving the foundation and the track.

Despite its looks, the gravel is well compacted and has very little chance of washing away without significant outside force.