In early September I was looking for a hike accessible by train, not too long and most importantly in an area which is very dark to try getting pictures of the milky way (better would be a few weeks earlier but it still worked well). That's why I also stayed over night but it's also doable as a day hike, just starting on time is needed.
Due to the rather low height this hike is throughout most of the year, except winter, an option and there are many variants how to combine everything.

Start: Station Küpfern; 2: Stallburgalm; 3: Burgspitz; 4: Almkogel; 5: Ennser Hütte; 6: parking lot Bambacher; 7: train station Großraming; Ziel: Gschwendtalm . Source: mapy.cz

Access public transportation
From Linz taking the train to Küpfern (direction Kleinreifling usually), it takes around 1.5h and is mostly without changing. The return is from the station of Großraming, just the one next to Küpfern.

Access car
Probably best parking at the parking lot Bamacher and doing a slightly different variant.

Hike
Arriving at Küpfern Bahnhof and following the direction of the train a bit till you can cross through a tunnel to the other side. There are also the first signs waiting for you. (Another option is a steeper and not that well marked trail to the other direction from the station which brings you more direct to the Burgspitz).

It's close to two hours (as always depending on speed) to the Stallburgalm (2), there I stopped for lunch.

Afterwards there are two options how to continue to the Ennser Hütte.
1) Via the Burgspitz (3) (and possible Almkogel (4)with a 30min detour) which I got recommended as it is supposed to be nicer, following the long ridge between Burgspitz and Almkogel. From the Burgspitz you have great views to the North towards the flat area around the Danube.
2) Or more straight to the crossroad a bit below Almkogel, this path is shorter but you do not have the nice ridge and peak.

At the crossroad both hikes meet again and there are the options either going straight to the hut, or better adding the Almkogel to your trip. Takes around 15min one way extra but rewards with great views to the Gesäuse and the Haller Mauern.

Down to the Ennser Hütte it is under half an hour. The most direct variant from Küpfern via Stallburgalm and the second hiking option should take around 3.5h.
I'm a very fast walker and needed for option 1 plus Almkogel also only 3.5h, you just need to figure out how your speed is in comparison to the official markings.

I then slept at the Ennser Hütte but if you started in the morning there's enough time to make it back down the the station in Großraming. Again two options.
1) the faster one is just straight down from the hut a bit via road and then through the forest to the parking lot Bambacher.

2) the more interesting one is going back up to the crossroad at the ridge you've already been before and go towards Burgspitz (especially if you've skipped it before this is perfect now), and just before the peak there's a sign leading you down again. If you came via Burgspitz und don't want to walk parts twice the first option is definitely better.

3) to the Gschwendtalm and taking a hiking taxi from there (or walking along the street/hitch-hiking, around 7km). You can also plan another overnight stay there, all, Stallburgalm, Ennser Hütte und Gschwendtalm, offer overnight stays so you can spend several days in the region and combine the hikes differently.

To shorten the hikes from the last parking lots to the train station, there is the option of the hiking taxi. If you stay overnight in the region it costs 1€ per km and person (therefore around 7€), otherwise you pay the normal fee which is around 20-25€ from the parking lots Bamacher or Brunnbach/Gschwendtalm to the train station. Taxi numbers and more info you can find here.

From the parking lot you then follow the street a bit till a parking lot where the hiking path separates again from the street and bringing you via forests and meadows more straight to Großraming in around 1.5h. Alternatively, following the road and especially in the afternoon there are high chances to be able to stop a car and get a ride towards the town. I needed around close to 3h for the descent from the hut via Burgspitz to the train station.

So my overnight stay looked like that:
Train from Linz 11:52 at to Küpfern at 13:38 (with Vorteilscard around 8€)
Then to Stallburgalm (around 15:00, official estimation is 2h though), Burgspitz and Almkogel (17:00) and down to Ennser Hütte.
The next morning (leaving hut bit before 10:00) towards Burgspitz again and then descending via the parking lot and then the hiking path to Großraming and catching the train at 12:23. Partly I was running though so either leave a bit earlier, skip the Burgspitz or just take the next train (either an hour later with changing or just two hours later and direct), many options.  

If you want to do it as a day trip I would recommend it like that:
Leaving Linz at 7:52 and arriving in Küpfern at 9:38. Around 11:30 at Stallburgalm, possibility of early lunch, then either straight to Almkogel or via Burgspitz the longer route to Almkogel (around 14:00 at Almkogel), then descending via the Ennser Hütte back to Großraming. One train to catch it would at 18:29, the last one at 20:29 (maybe bring headlamp!) back to Linz.

And finally some photo results of the milky way over Almkogel: