Rail passengers face travel disruption as Silverlink train drivers stage a walkout for the third consecutive Friday.
Members of the Aslef union have been taking part in the industrial action as part of a row over pay and pensions.
Silverlink said only a limited number of trains will be in operation from a limited number of stations and urged passengers to use alternative routes.
The company runs services out of Euston railway station in north London.
'Not to travel'
Silverlink services are only running twice an hour between London Euston and Tring in Hertfordshire.
As a result there are no services through Cheddington, Leighton Buzzard, Bletchley, Milton Keynes Central and Wolverton.
The Bedford to Bletchley line is also closed for the day.
Silverlink said: "We are strongly advising passengers not to travel.
"If your journey is essential then please arrange to use other means of transport."