One train is a step towards multiple trains per day testing it. If there are issues with the test train with signal sightings or something, no other trains get tested. If it works though on the test train(s), see if other trains can be done. In some cases, yes. There are many places though where service levels are much lower (half hourly or less) and any speed increase would lead to journey time reductions. Increasing the speed through Deansgate to Piccadilly will do sod all for anyone. The North Wales Coast though (The line works for the example, not saying any improvements could be made here) has trains fairly well spaced out and any speed increase wouldn't lead to fast trains catching up to slower ones. Everything is complex, doesn't mean it is impossible.