I hope this forum is being of help to you.
By-the-way - most people on here will distinguish between a 'locomotive' or 'engine' which is the machine (usually steam powered on here), and the 'train' which is a set of vehicles, either passenger coaches or goods tracks and vans, which is hauled by the locomotive!
(Of course, just to be difficult the whole thing, loco and coaches, can sometimes be referred to as a 'train'.)
The D17 (NER 'M') is probably a bit closer in looks to a D20 (NER 'R'), the main difference being that the 'R' had a slightly larger diameter boiler.
From the point of view of the murderer (and the victim) the coaches would have been more significant than the locomotive. Most railway companies (including the North Eastern Railway) did not encourage visitors and/or passengers to ride on the locomotive! Not that that it didn't happen of course, but drivers and firemen would not want to be reported to Mr Vincent Raven, who could be quite fierce!
On almost all steam-hauled trains it was not possible to move from passenger coaches to the locomotive, at least without desperately clinging to the outside!
Of more significance would be whether the train was made up of 'corridor' coaches, where there is a connection for people to move between the vehicles, and 'non-corridor'.
The line from Newcastle to Alnmouth is part of the main-line route from Newcastle to Edinburgh, so some of the fast main-line trains on the line would have been made up of corridor (or to be fussy 'vestibuled') vehicles. However, many of these would not have stopped at Alnmouth, let alone Widdrington which was I believe Mr Nisbets destination. In the first part of the 20th Century there were a large number of local services, stopping at smaller stations. Alnmouth was the junction station for Alnwick, and had it's own engine shed, and locomotives based there worked a lot of local trains, both to Newcastle and to Alnwick.
I've just checked my library, and the NER Northern Division Passenger Working Timetable for 1908 shows a 10:27am Newcastle to Alnwick, arriving at 12:20pm. Is this your train? If so it was a local train hauled by a locomotive stationed at Alnmouth Shed. This gives rise to to two further points.
In 1910 the R class was still very much a main-line engine, and unlikely to be used on a local stopping train, unless it was 'filling in' between other duties.No 'R' was stationed at Alnmouth until 1938.
The second point is that this train (assuming it's the right one!) was probably not made up of corridor vehicles. The majority of NER local trains at that time were made up of non-corridor stock. The best of these were the 52' Clerestory vehicles such as the one based at Beamish:-
Each coach was divided into separate compartments,each with a door on each side. In most coaches there was no way to get from one compartment to another. Some of them however, did have an internal corridor allowing passengers riding in the different compartments to have access a lavatory. Passengers not in such coaches just had to cross their legs!
There were still a lot of older coaches in service in 1910, and some of the examples at Tanfield may give a better idea of these.
Most preserved railways use ex-British Railways corridor vehicles with lavatories - which will not give quite the same experience!
This forum does have a lot of knowledgable people, so someone may have more information than I do, but I'll look through my library some more.
I presume you have seen these articles:-
.... from the British Transport Police...
.... more discussion......
.... Evening Chronicle
(This makes reference to a book available on Amazon.)
By-the-way, I volunteer at Shildon, so if you PM me I might be able to help with access to the D17.
