The most difficult engineering problem that Isambard Brunel had to solve when building the London to Bristol line was the Box Tunnel. Positioned between Bath and Swindon, Box Hill consists mainly of limestone.

Two miles long, on a descending gradient of 1 in 100, one MP warned that the slope was so great that if the brakes failed, a train would leave the tunnel at 120 mph (193 kph). Another MP argued during a debate in the House of Commons on the subject, that if the tunnel was built, no one would be brave enough to enter it. The tunnel was so straight that it was argued that on some days the "sun actually shines through the complete length of the tunnel".

When completed in 1841, Brunel claimed it was the longest tunnel in the world. In fact the Sapperton Tunnel on the Thames & Severn Canal was about 500m longer and 52 years older than the Box Tunnel.