Services to the country's busiest railway station were disrupted on Monday morning because of track and signal failures, hours before MPs take evidence on rail infrastructure.
South Western Railway passengers faced travel chaos because of a track circuit failure at London Waterloo.
The station was partially closed last summer because of an £800 million upgrade.
A signal failure at Woking compounded Monday's disruption.
The Rail, Maritime and Transport (RMT) union said a Transport Select Committee hearing on rail infrastructure on Monday must tackle "underfunding" of maintenance, renewals and upgrades as well as addressing the issue of outsourcing and casualisation in the wake of the Carillion collapse.
General secretary Mick Cash said: "The backlog of maintenance works on Britain's railways means that passengers are paying a heavy price for years of under-investment as? our creaking rail infrastructure is stretched to the limit.
"On top of that, safety-critical renewals budgets are under attack, leaving crucial works, and the workforce that delivers them, on a constant knife-edge.
"That stop-start approach at a time of surging rail demand has to end and we need long-term planning and stability free from the kind of risks we have seen from the Carillion collapse.
"That kind of short-termism is amplified by Transport Secretary Chris Grayling's decision to shelve essential electrification works.
"That scandalous move has already thrown long-planned initiatives, and our members ?who had been hired to make them happen, into total chaos. Those works should be reinstated.
"We need a publicly-owned railway free from the profit motive where planning and investment come first."
Passengers were told that one line in the London Waterloo area was blocked.
As a result, three platforms had limited access, meaning that some services were cancelled, delayed, or subject to short-notice alterations.