I enjoy the Railway Detective series and I've read them all through. This is much of the same, and I enjoyed the story. However this far in there are some glaring problems with Marston's writing.
The most obvious is the use of Deus Ex Machina plot resolution points. For e.g. in this one, without spoiling too much, the plot is resolved in the last 10 pages of the book by the chance discovery of some letters, the contents of which have never even been hinted at before. The discovery is made 'off-camera' as well. There is also the usual sense of rushing, in every Railway Detective book it's very much "And they found the murderer, the end!" All the exciting trials and arrests happen, again, 'off camera'. Marston prefers to tell rather than show when it should be the other way round.
That said, I finished the story and I enjoyed it, but I can't help feeling the author is somewhat running out of ideas. Apart from anything else, this plot is only tangentially related to railways at all (and again, that tenuous link is brought in in the last chapter as an "oh by the way, the dead man had xyz link to the railways").
If you've read the others and enjoyed them, by all means get this one too, the story is still good (if overly familiar by now!) and the pace keeps up.