UK Rail and Trains

Exactly! Tunnel capacity is a hard upper limit, but in an ideal world you'd be using every minute of that capacity for either passengers or freight. Depot capacity issues are very solvable.

Station capacity is also an issue (particularly passport control issues at St Pancras, which limits how many passengers they can process even beyond platform capacity), but that's relatively solvable too. One of the competing proposals (possibly the Gemini one?) wanted to make Stratford International its terminus rather than St Pancras to avoid station congestion. If HS1-HS2 link were back in play, that would open up other options for alternative termini too (such as Old Oak Common, or HS2 stations out of London).