Have a 9 hour layover in Doha: 17:40 to 2;20 AM. Even though we will have lounge, may want to leave and see Doha for 1-2 hours, apparently only 15 minutes by train.
I assume we will have to go through all the departure processes again when we return to airport.
Is it worth it?,especially at those hours? If so, what to do for 2 hours except walk around and see the impressive architecture

Maybe it’s because I utterly loathe Doha, but if I were you I’d rather invest money in an airport hotel room (there should be one airside?) and sleep it off.

Curious - why do you loathe Doha? I am not saying I will appreciate the culture there but it is always interesting to see something different.
Yahoo had a click bait yesterday on how over the top life in Dubai is (people welding together 2 hummers to give them a ten foot wide vehicle, gold plated Rolls Royces. etc.) can't find the link now.

We can sleep in the lounge, even though it is described as the cathedral of snoring

Presumably you will already have your boarding pass for the second flight and your bags (if any) will be checked through? Then there isn't much difference in the airport procedures if you go out or not. Rarely you may get to skip security when in transit. If you're in economy there may be long queues for transit security, not so much when entering from landside.

Now that the train is in operation you don't need to mess around with buses or taxis. I would happily spend 2 hours riding the train back and forth, if you wish you can also walk around the Corniche and Souq Waqif, although it's all rather same-y. The Museum of Islamic Art is often recommended but is currently closed. Personally I like walking around suburbs and the worker neighbourhoods, although it can be a bit intimidating the first time and maybe not recommended in the dark as it will be on your schedule.

I would go out of the airport for 2 hours. I actually like Doha better than Dubai. It's all a personal thing. But, if they have a train now, I would definitely go. The taxi drivers are clueless including the bus drivers. Use a good offline map app like HERE WE GO and it will get you around pretty good after you download the city. Better than google offline maps.

Doha is one of the airports where a quick trip into the city is feasible alternative, in terms of logistics and cost, to an extended stay airside. And for me a bit of an adventure and a cultural experience, however minor, beats free food and drink and in a comfortable-but-sterile environment. If you do crave a little of the latter, the flights either side of Doha should provide it

But 2 hours will be eaten up by the faffs involved in getting out of the airport into the city and then back. So it's pretty much an either/or decision.

If you opt for the city trip, I'd suggest heading out of the airport as soon as you arrive. You can then weigh up how much you are enjoying your adventure and tailor the return to the airport, and the delights of the lounge, accordingly.

Getting to the airport metro station isn't much of a challenge if you follow the signs. Tickets are cheap at OMR2 (around EUR0.5), though it's far better to get a day pass, costing equivalent to three trips and saving the faff of buying more tickets. You can get to the souq with a change from Red-line to Gold at Msheireb; do your stuff, shop and dine, or maybe walk through the souq to the corniche to get a view of the city.

If you prefer Mall-life you can go straight through on the Red line from the airport to DECC (Doha exhibition and convention centre) for City Centre Mall, in the middle of the West End skyscrapers.

If time is running short, a taxi/uber back to the airport isn't going to break the bank.

A couple of flies in the ointment: I read somewhere that immigration isn't always happy about OK-ing short visits into the city between flights; and it would be useful to be able to ditch your hand luggage. If you have significant baggage, maybe a quick trip to the lounge on arrival to park it there is the answer. Then of course the elephant in the room is satisfying whatever Covid-related requirements are in force when you visit.

I personally enjoyed my visit to Doha when I was there over New Years, definitely more than I enjoyed my time in Abu Dhabi. The airport is close enough to the city that a 2 hour visit is definitely doable, and I'd certainly recommend it if you have the time!

This historic thread on the Qatar forum, obviously.pre-pandemic and before the metro service to the airport was available is probably worth reading...

QR free transit visa - DIY Doha night tour?

Are you arriving from a Green or Red list country? Red list country requires hotel quarantine.