Industrial action is disrupting Midland Mainline (MML) rail services for the second consecutive Friday.
Passengers will have a limited service on three more Fridays
It means emergency timetables and fewer trains on the route from London St Pancras through Beds, Northants, Leics, Notts and Derbys to Sheffield.
Guards are striking in a dispute over safety due to hit services for four Fridays and leading to crowded trains.
MML said it is doing all it can to limit any disruption caused by the absence of guards.
Silverlink, Thameslink, Wagn, Northern, Central and Virgin Trains are accepting MML tickets on Friday, or passengers can use MML services on Saturday.
Derby-based MML said in a statement: "If passengers choose not to travel they can claim a full refund on their ticket.
"We understand that this may cause some disruption to those wishing to travel and for this we sincerely apologise."
The RMT union said MML is compromising safety by not putting two guards on each train because staff cannot move through different sections of the train when it is moving as they are sealed off.
Ken Usher from the union said: "We are concerned that the public get to know what this is all about.
"It's not a pay issue, we're not being greedy, we're not asking for a penny-piece more, it's protecting their interests, our own interests and protecting the services that they would be travelling on.
"The managing director needs to get his finger out and get round the table, the ball is in his court, we're able to talk."
The firm said a risk assessment, by an independent company, found this would be safe.
Strike action is also set for 10 June and 17 June.