Train schedules include what's know as "recovery time" in the journey, to allow for delays caused by slowing down for engineering works, etc, to be made up.   It's a long standing practise and it does help avoid long distance services getting progressively later on their journeys.   Very often, though, the recovery time is not actually needed and trains arrive early outside busy stations and have to wait to get in, or arrive early and sit and wait before continuing.   If it would save fuel to have the drivers go just a tad slower over these recovery stretches, why not?   And, as a double plus it won't annoy the passengers who are stuck in the train that's waiting for its platform, or who are sitting in the train wondering why it's wating so long before carrying on.