Workers heading to London Waterloo are being warned that a severely reduced timetable is in use for three weeks as almost half of the platforms at London Waterloo are closed for an Â£800 million overhaul.
South West Trains predicted that stations such as Vauxhall, Clapham Junction, Woking and Guildford will be "exceptionally busy" and customers could be forced to queue to get in, but no serious issues were reported on Monday morning.
A signalling problem affected trains entering three of the operational platforms at Waterloo, leading to delays of up to 20 minutes.
Network Rail chief executive Mark Carne told the Press Association: "It's gone very well this morning. So far, so good.
"This is the start of a three-week campaign and there will be some difficult days I'm quite sure.
"But passengers I think understand that this is a necessary evil to achieve a fantastic improvement in the services that they are going to enjoy for decades ahead."
Seven stations in south-west London will be closed for at least some of the project.
An average of 270,000 journeys are normally made to or from Waterloo every day.
The project to extend the station's platforms will allow longer trains to operate on suburban routes from December and provide space for 30% more passengers at peak times, Network Rail said.
To ease some of the impact of the work, the old Eurostar platforms are being used atWaterloo for the first time since the cross-Channel train service relocated to London St Pancras in 2007.
Rail chiefs urged passengers to consider taking a holiday, working from home or travelling earlier or later than normal while the project is under way.
But business leaders said this was not always possible, and the Federation of Small Businesses warned traders will suffer lost income they will be unable to recover even once the work is completed.
Mr Carne said: "What we've said is that if you have the flexibility to change your travel plans, you might want to consider it.
"Of course it's not available to everybody and that's why we are continuing to run a service."
First Group and Hong Kong-based MTR will take over the South West Trains franchise from Stagecoach on August 20.
David Sidebottom, director of passenger watchdog Transport Focus, said: "How this work is managed over the next four weeks is key to passengers' trust in the rail industry.
"They need to know when the work is being done, what it means to them and their journey, and they should be appropriately compensated."
The Waterloo work will culminate over the August bank holiday weekend, when it will be one of a number of large projects being carried out.
Passengers hoping to take a leisure trip on the final public holiday before Christmas will find major changes to services out of London Bridge, London Euston, London Liverpool Street and London Paddington, as well as Waterloo.
The work at Euston will be one of the first major physical projects in preparation for the HS2 high-speed railway, as a new power supply is installed at the station.