Rail strike: Grant Shapps dismisses calls for government involvement as a 'stunt'
Only 20% of trains are understood to be running during strikes as Boris Johnson urged commuters to "stay the course".
13 rail operators walked out at midnight.
Further strikes are planned for Thursday and Saturday.
The RMT union is asking for a pay rise of at least 7% to offset the rising cost of living
but it says employers have offered a maximum of 3% - on condition they also accept job cuts and changes to working practices.
CEO of Network Rail, Andrew Haines, said the right deal "we could go above" the 3% pay rise.
Trains that do run will start later and finish much earlier than usual - between 07:30 and 18:30.