The RMT has announced strikes on Monday, March 26 and Thursday, March 29 - with Northern services in Sheffield and Doncaster once again being targeted ahead of the busy Easter period.
A Northern spokesman said: "The majority of available trains will operate between 7am and 7pm as we focus on running as many trains as we can to get you into work and home again.
"As the overall number of trains running will be reduced, we expect trains and any replacement buses we operate to be extremely busy. Please allow extra time for journeys, plan carefully and consider whether travel is necessary."
A statement added: "During the RMT strike action we expect all services to be busy, especially in the morning and evening peak periods, and advise you to allow extra time to travel."
The strikes are the latest in a long-running dispute with the RMT and Northern with the union battling to keep guards on trains.
You may be able to use your ticket on other train operator services.
You will be able to use tickets on Arriva Bus services (including Yorkshire Tiger) to make journeys you would otherwise have made by rail.
Further details and strike timetables are available HERE