West Highland Line - Glenfinnan

From Fort William to Mallaig via Glenfinnan the steam train and the ordinary diesel train from Glasgow run over exactly the same tracks.

And it is almost impossible to do the steam train and return from Glasgow in the day, unless you travel from Glasgow to Fort William in one direction or the other in the seated car of the Caledonian Sleeper from/to Glasgow Queen Street Low Level.

To do this (on a weekday) you would need to leave Glasgow Queen Street (High Level) at 0822, arriving at Glenfinnan at 1249. The afternoon Mallaig bound steam train stops at Glenfinnan Station from 1331 to 1356 (so you will have time to look in the Station Museum before they arrive and invade the Museum).
As soon as the steam train leaves go and have lunch in the station Dining Car (an old Railway Restaurant Car).

(Do it the other way round, lunch first, museum after the steam train and you get free museum entrance).

You then have time to go and see the Jacobite Memorial at Glenfinnan before going to the viaduct for the eastbound steam train at 1519.
Your return train to Glasgow leaves at 1654, arriving at 2125.

At the same time the afternoon diesel train to Mallaig leaves from the opposite platform at Glenfinnan.

What you could also do is to return to Glasgow via Glencoe- the service bus #500 leaves from Glenfinnan at 1640 Monday to Friday, arriving at Fort William at 1710. The bus to Glasgow via Glencoe leaves at 1740, arriving at Glasgow at 2051 (so before the train).

The connection train to bus at Fort William is very tight, hence taking the bus from Glenfinnan. But you could equally take the train back to Fort William, have about 90 minutes in Fort William then take the 1910 bus back to Glasgow via Glencoe arriving at 2216.
Doing it that way you will see the Caledonian Sleeper train arrive in from the sidings at 1850.

You could of course just stay on the diesel train to Mallaig and spend the afternoon there (including seeing the first steam train on the platform there and then departing at 1410, and the second steam train arriving at 1506)

And if you would like to have lunch at the Dining Car as Stuart recommends, I would highly recommend reserving in advance. It is very small — and one of the very few options in Glenfinnan- and run by a wonderful couple,of whom the husband is a great chef — so it books up quickly !!

Huge kudos to Stuart for pointing out all the details of how this could be done in a day.

I will point out though that the loch on which Glenfinnan sits, Loch Shiel, is absolutely gorgeous and it is great to stay overnight so you can take advantage of a boat ride out on the loch. Check out www.highland cruises.co.uk. The views are stunning and Ali is great at pointing out all the wildlife — especially eagles ! — around the loch.

If doing selfies next to the train will substitute for visiting Glenfinnan, the steam train is on the platform in Fort William prior to departure and last time we were there, you didn't need a ticket to stroll the platform, touch the engine, and take a selfie in front the the "9 3/4 Hogwarts" plaque. We did that then drove to Glenfinnan and made it in time for the crossing, although it was close.

The train station is west of the viaduct and unless you arrived on the diesel train and waited at the viewpoint for the Jacobite, it wouldn't work. The trains pass each other depending on the delays, etc. on the single track with short sidings and that's an opportunity for a blurred photo.

If doing selfies next to the train will substitute for visiting
Glenfinnan, the steam train is on the platform in Fort William prior
to departure and last time we were there, you didn't need a ticket to
stroll the platform, touch the engine, and take a selfie in front the
the "9 3/4 Hogwarts" plaque. We did that then drove to Glenfinnan and
made it in time for the crossing, although it was close. The train
station is west of the viaduct and unless you arrived on the diesel
train and waited at the viewpoint for the Jacobite, it wouldn't work.
The trains pass each other depending on the delays, etc. on the single
track with short sidings and that's an opportunity for a blurred
photo.

None of that is true. When the trains pass they pass on the opposite platforms at Glenfinnan station. Since 2022 the correct information about Fort William Station now (as in last year) has been given several times. The platform at Fort William is now barriered off prior to departure. Driving to Glenfinnan doesn't seem to be an option for you as you intend to reach there from Glasgow by public transport.
They do that for the morning departure and again in the late afternoon.
The timetable is as it is because after Glenfinnan the next passing loop is at Arisaig, then at Mallaig Terminal station. Glenfinnan and Mallaig are where you will now get 'selfies'. Before Glenfinnan the previous passing loop is at Fort William.