A 1914 rail tunnel from Hamburg to London

How would a surprise rail tunnel between Hamburg and London affect the First World War?

Its limited train throughput, turnaround problems, steam exhaust, and vulnerability to gas or mining make it a dangerous breach but not an easy German invasion route.

The sudden Hamburg-London connection starts with a deliberately impossible premise and then runs into transport constraints. Alex assumes limited turnaround arrangements and at most one train per track, leading to only a few trainloads arriving over many hours. Those restrictions are assumptions about the imagined tunnel, not a general rule for railway operation. Steam and smoke disposal create another unresolved problem. The closing remarks add the route's vulnerability to gas and military mining, without treating it as a straightforward means of delivering an invasion force.