Why don't trains make *that* sound anymore?
Modern train tracks are welded into seamless rails to eliminate the classic 'clickety-clack' sound and allow faster travel. However, this creates a thermal expansion problem, which is solved by embedding tracks in jagged rocks that interlock and resist buckling. The shape of the rocks is critical — smooth rocks would fail to hold the tracks in place.
Summary
The video explains why modern trains no longer produce the iconic 'clickety-clack' sound that older trains were known for. Historically, train tracks were laid in sections with small gaps between them, which allowed the steel rails to expand and contract with changing temperatures. The sound was a byproduct of train wheels striking those gaps. However, those gaps also acted as a speed limitation, so modern engineering replaced segmented tracks with continuously welded rails — eliminating both the sound and the speed restriction.
This solution introduced a new engineering challenge: without gaps, there is nowhere for the steel to expand when heated. The internal pressure that builds up in welded rails can become so intense that the tracks can violently buckle out of alignment, rendering them dangerous or unusable.
To counteract this, modern tracks are fastened to sleepers (also called railroad ties) and then embedded in a bed of jagged, angular rocks called ballast. The irregular, jagged shape of these rocks is the key to their function — they physically interlock with one another, generating enormous friction. When the rails expand and push outward on the sleepers, the interlocked rocks resist the force and hold everything firmly in place, keeping the rails straight even under significant thermal pressure. The video emphasizes that this design detail is non-trivial: if smooth, rounded rocks were used instead, the vibrations from passing trains would cause the rocks to shift and resettle, gradually losing their interlocking grip and allowing the track to buckle.
Key Insights
- The classic 'clickety-clack' sound of trains was caused by wheels hitting gaps deliberately left between rail sections to allow for thermal expansion — not an incidental design flaw.
- Modern tracks are welded into a single seamless rail to eliminate speed-reducing gaps, which is why the clickety-clack sound no longer exists on contemporary rail lines.
- The elimination of expansion gaps in welded rails creates dangerous internal pressure buildup from heat, which can cause rails to violently buckle if not managed.
- The jagged, angular shape of ballast rocks is functionally critical — their irregular geometry causes them to physically interlock, producing the friction needed to resist rail expansion forces.
- Smooth, rounded rocks would fail as ballast because train vibrations would cause them to shift and lose their interlocking grip, allowing the track to buckle over time.
Topics
Full transcript available for MurmurCast members
Sign Up to Access