Trains travelling between Weymouth and Poole are facing disruption.
Due to a signalling problem caused by multiple axle counter failures between Poole and Weymouth, all lines are blocked.
Train services running to and from these stations may be delayed or revised.
The disruption is expected until 6pm.
In a statement, South Western Railway said: "We have been informed of multiple axle counter failures between Poole and Weymouth. This means that trains are unable to run between these stations.
"Until further notice, the following alterations will be necessary: Trains between Waterloo and Weymouth will run between Waterloo and Poole in both directions."
Tickets will be accepted at no extra cost on the following services: Great Western Railway between Dorchester and Weymouth in both directions First Bus routes 10 and X54 in the Weymouth and Dorchester areas Morebus routes 1, 16, 17, M1, M2, 8 and 9 in Bournemouth and Poole areas.
Buses have been arranged to run on the following routes: Bournemouth to Weymouth in both directions, buses are expected to be in place from 2.15pm.
The spokesperson added: "For further information or onward travel advice please speak to a member of staff or use a station help point.
"We are very sorry for any delay that this may cause to your journey."
What is an axel counter:
An axle counter is part of the signalling system. There are at least two axle counters per signalling section; one to count the number of axles on a train entering a signalling section and one to count the number of axles leaving a signalling section.
If these two totals match, the section becomes clear for the next train. However, if the totals do not match or if there is another issue, the section of track remains occupied, holding the protecting signal at red. When there is a failure trains will stop before the affected signal and will have to be verbally directed through the affected area by the signaller.
When being talked through the section by the signaller trains will run at a reduced speed. The process of trains being verbally directed through affected signals can add an additional ten minutes per signal to the journey time.
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