Train passengers have been warned of "significant disruption" to services as workers strike over guards on trains.
Rail, Maritime and Transport union members have walked out for the first of seven strikes on Saturdays before Christmas.
The RMT general secretary Mick Cash said action was "solidly supported" by members.
West Midlands Trains said it was running a reduced service and was committed to safety.
The action affects services in the West Midlands and on London Northwestern Railway.
The National Railway Enquires website warned passengers only a "very basic service" would run, with some stations not being served at all.
"Please carefully consider whether your journey is absolutely necessary, as significant disruption is expected," it said.
The RMT said the strikes were "over plans by the company to bulldoze through driver-only operation".
Mr Cash said: "RMT members are standing rock solid and determined this morning across the West Midlands Trains franchise as we fight to put the safety-critical role of the guard at the platform/train interface, and the safety and accessibility of the travelling public, before the profits of the train operator."
West Midlands Trains said it was determined to resolve the dispute.
A spokesman said: "There is no need for this action. We have never proposed driver-only operation. We are committed to keeping a safety-critical conductor on every passenger train."
The firm added it had proposed a solution on its new trains which would see conductors "retain responsibility for safe dispatch from stations".
It is running a reduced service using managers who have guard competency.
Birmingham's NEC said its events will go ahead and advised visitors to plan journeys carefully and consider alternative transport.
Follow BBC West Midlands on Facebook, on Twitter, and sign up for local news updates direct to your phone.