Q.Explain, in terms of thermal expansion of solids,

1. Gaps in railway tracks: A steel rail heated on a hot day expands along its length. If successive rail lengths were laid end to end with no gap at all, this expansion would have nowhere to go, and the rails would be forced to buckle sideways out of the straight line of the track -- a serious safety hazard. A small expansion gap at each joint gives the rail room to lengthen into on a hot day, closing up as the rail expands, without pushing against the next length of rail.
2. Rollers under a bridge: A long steel or concrete bridge span expands and contracts along its length as the temperature rises and falls through the seasons and through the day-night cycle. If both ends of the span were rigidly fixed to their supports, this repeated expansion and contraction would set up large internal stresses in the span and could crack or damage the supports over time. Mounting one end on rollers (or another sliding/expansion-joint bearing) lets that end of the span slide freely as the bridge lengthens and shortens, relieving the stress that would otherwise build up.
   ✓Final answer

   Both the track gaps and the roller support exist to safely accommodate the thermal expansion (and contraction) of the long metal structure, preventing buckling or built-up internal stress.

Recognise that a long, rigidly constrained solid structure has nowhere to go when it expands, and that engineered gaps/rollers deliberately provide that room.

• Attributing the gaps/rollers to vibration damping or to allowing for construction tolerances, rather than to thermal expansion specifically.
• Thinking the gap is meant to stay open at all temperatures rather than to close up as the rail expands.