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  Cancellations to services between Basingstoke and Exeter St Davids
Due to more trains than usual needing repairs at the same time between Basingstoke and Exeter St Davids fewer trains are able to run on all lines.
What's Going On:
Train services running through these stations may be cancelled, delayed or revised. Disruption is expected until the end of the day.
What We're Doing About It:
Yesterday, we were informed that many of our diesel fleet needed repairs at the same time. Unfortunately this issue has continued into today.
Until further notice, the following service alterations will take place.
- Services between London Waterloo and Exeter St Davids will run between Salisbury and Exeter St Davids only. Trains may have to run at reduced speed between these stations and due to the single line sections along this route trains may be delayed awaiting to pass a train in the opposite direction.
- A shuttle service will operate between Salisbury and Basingstoke
If you require a train between London Waterloo and Basingstoke, you are advised to board one of our services between London Waterloo and Weymouth, Poole and Portsmouth via Eastleigh or a stopping service to Basingstoke.

Paul Clifton reporting that it's precipitated by an issue with biofuel content.

Mark

Expecting older diesel engines to be happy to run on fuel with added vegetables may have been a tad optimistic..

    I wanted to make you aware of an issue currently affecting our diesel
fleet, operating on the West of England line between Basingstoke and
Exeter and between Southampton and Salisbury via Romsey.

After becoming aware that some of our trains running on this route have
not been performing as well as expected, we then discovered a problem
with the fuel that these trains use. This is causing performance issues
and means they cannot travel at their usual speeds. Trains that have
reported problems are being taken out of service for inspection and the
replacement of engine filters.

In order to be able to accommodate these additional inspections and
repairs, and to try and minimise short notice delays and cancellations,
our control team has had to reduce the timetabled frequency of services
as of today (Friday 21st April) until further notice. A summary of the
changes are as follows:

   * One train per hour will operate between Basingstoke and Salisbury
   * One train every two hours will operate between Salisbury and Exeter
St Davids
   * One train every two hours will operate between Southampton Central
and Salisbury via Romsey
   * Services to Westbury and Yeovil Pen Mill will not run

We expect to face this issue for several days as investigations continue
and engineering mitigations are put in place. We will of course provide
a further update with more information when we can. I would take this
opportunity to apologise for the delays and disruption that will still
be caused despite the mitigations we are taking.

Wouldn't it have been an idea to test it before using it?

 One of the SWT fleet of Class 159s, 159 007, was chosen to take part in a Network Rail bio-fuel test in 2008 [2]. It ran for several months on a mixture of diesel, soya bean and oil seed rape oil. The fuel mixture did not affect performance or cause any damage to the engines of the unit though was found to be more expensive than conventional fuel.

The issue appears to be related to bacteria in the bio-fuel that appears to be impacted by warmer weather and it can affect the fuel tanks in the depots and fuel filters and warming tanks on units. Fleet are aware of the problem and samples have been take and sent for analysis to identify the particular type as this is important in how it is irradicated. In parallel to this they are trying to understand the source of the contamination. Short term the problem can be mitigated by changing the fuel filters and if necessary, the cleaning the fuel warmers. Fleet are working on a plan to understand the status of all the stock and over time how frequently the they need changing / cleaning.

Train plan for tomorrow issued at 2030 this evening with basic principles 2 hourly service Exeter - Sal, hourly Salisbury - Basingstoke, 2 hourly Romsey Rounders and all services via the Western region cancelled.

Different engines? Use of biodiesel, rather than standard diesel mix?

   On Thursday 27th April, most trains between Exeter St Davids and Waterloo will run as scheduled however some will start or finish at a different station to usual. Journey planners have been updated to reflect this.

https://www.gazetteandherald.co.uk/news/23486359.south-western-railway-train-timetable-reinstated-diesel-problem/

Neil Drury, SWR’s engineering director, said: “We’re sorry for the disruption that customers in the Basingstoke, Exeter, Romsey, Salisbury and Yeovil areas have felt since last Friday.

"It was a difficult decision to amend our timetables and we have worked hard to restore services as quickly as we can.