Why not have where it can be muti-player ? Where you can join a friend online. One could be engineer the other the conductor or fireman, or even control the dispatch and signals ?
When tsw3 released, multiplayer was cancelled cuz dtg want to focus on singleplayer and multiplayer requires too much moving parts and causes more problems since the game isn't built around mp. In the thread below, if you scroll down a bit, there is an official statement by a dtg staff member. https://forums.dovetailgames.com/threads/multiplayer-for-tsw.57489/
Manual Firing for steam engines is currently in development and has been since the release of Spirit of Steam back in May. I believe that Guard mode has been considered but I have no info on that. When it comes to Signalman, don't think that would work and plus, most of the routes currently in the game (and coming soon) are set in the 21st century when signal boxes are becoming rarer due to automatic signalling
Experience of multiplayer in Run 8 and SimRail not good, IMHO. Not sure if it's still the case but SimRail rapidly filled up with griefers and anarchists, intent on wrecking the experience for other players. It became a glorified chat room, though not that chattly last time I was on there. TSW doesn't even have the architecture to allow real players to take over the signalling side of things. Run 8 even worse. Placed a train, drove it a little way and parked it up out the way. Next time I came back it had been moved or deleted. Also found it very much a "good ole boys" club with regulars chatting and the newbie ignored. Far more priorities for DTG to look at IMHO, dynamic signaller/despatcher with random events would be far more fulfilling than MP.
I think it's been fairly middling on the "what do players want?" scales when DTG put out their surveys, and to be fair many people don't want to rely on someone else for their play. Personally I would find being a guard on a HST boring in game because there's nothing to do, and even at station stops you'd open the doors, then close the doors. Hardly epic gameplay. Not sure what you mean by "join a friend online" and how that would make a difference in gameplay? If you want to have it that you and a friend play on the same line then how would this work? Unless said friend was on the train in front and deliberately drove slow so you got adverse signals it would not impact what you do at all... As OldVern says above, the MP experience on other sims hasn't been favourable with crashes or people just being idiots and making it hard for others to actually play the game. I believe there was one snarl up with resulted in people sitting at red lights for 90 mins whilst the signaller tried to unsnarl everything... Not the best experience as a player, unless your intention is to sit "on the radio" and chat about the red lights for the time it takes to watch an entire movie...
Regarding joining a friend online is coop mode. Both could be drivers doing their own service on the route or one could be driver and the other guard. Or could simulate yard workings, etc.
Gameplay with a party of people. I mean other games have coop where both of you can play together. In terms of simulating a guard, the host chooses a train and service where it has guarding functionality with their friend in the same party. The driver just does driving things. The guard is responsible for opening doors. Many of the uk locomotives and emus have guard panels which can be used by the person simulating the guard. It still needs to be built of course but it's not like there's zero gameplay.
From your example... I am the guard on a GWE service from Paddigton. I open the doors, wait 1 minute, close the doors. DO NOTHING FOR ALMOST HALF AN HOUR The train stops, I open the doors, wait 1 minute, close the doors. Sounds like no gameplay to me (I am biased by the way, I hate multiplayer where there's no direct interaction (such as fighting or driving games where you're actually against the other) with a passion... too much not actual playing / controlling)
