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).
A social network needs enough users to actually function. In the early days, Lemmy/kbin/associates were too quiet to be appealing, so there was a constant push to bring in new users. As this is a Reddit clone social network, inevitably that means hoping that Reddit users will come across.
I would argue that Lemmy et al is already at a high enough number of active users that there's a basic critical mass; that there's enough activity here such that a new user would find plenty to keep them engaged. It could certainly stand to be much bigger still, but the pressure to grow is much less intense.