Tragedy on the Branch Line is the most recent (2021) in Edward Marston's Railway Detective series. It has dropped to £1.89 at Kindle UK as of right now. No idea how long it will stay that way...
Kindle UK:
https://www.amazon.co.uk/dp/B08GPLRLRR
Kindle UK/Smile:
https://smile.amazon.co.uk/dp/B08GPLRLRR