We did the Indian Pacific from Adelaide to Perth across the Nullabor Plain in 1986 as Halley’s Comet was passing, primarily to get the least amount of light interference. Checking their website it says the trip is 2 days/nights which sounds about right. I was 11, it was long whatever it was.
The longest journey that I’ve taken is officially 53 hours (12510/Guwahati - SMVT Bengaluru SF Express (PT) - Guwahati to SMVT Bengaluru NFR/Northeast Frontier Zone - Railway Enquiry) but we were on it closer to 70 because of a breakdown in West Bengal where we sat overnight in the middle of nowhere while waiting for a replacement engine, or possibly track replacement. Can’t really remember at this point.
Probably the same here for single trips. I’ve had journeys that have required multiple transfers that lasted in the 24-36 hour range, but those usually include sleeping at the train station because we got in on the last night train, and the morning train didn’t leave until six hours later.
Trains are just so much better than buses. At least you can get up, stretch, walkaround, have a smoke in between compartments (well, back when I used them you could), have a drink in the drink car (if you’re lucky enough to have one.)
The distance wasn’t all that far (Edinburgh to Liverpool), but it took almost an entire day. It seemed like we were on the train(s) forever. The last leg of the trip, we transferred to a train full of drunken footie fans on the way home from a match–that was fun, seeing as how we were completely spent and they were all jacked up and rowdy as hell.
Madrid to Sevilla
We left around 7:30 in the morning, got to Sevilla around 9:30, 10:00 at night.
This was for a semester abroad. I was surrounded by other students my age (although they were complete strangers) and soldiers. The windows were open, you could stand out in the aisle and smoke, and one of the cars was converted into a snack bar. It was for the most part very enjoyable.
Careful, there…
I don’t know how many hours, but three nights, from Osceola IA to Seattle. I didn’t know that the trip would have taken half that time if I’d boarded at MSP and taken the northern route. Had a roomette so it was comfortable, but expensive ($1100).
About 51 hours, from Toronto to Edmonton. The trip should have taken 48, but there were a few delays along the way that caused us to be three hours late into Edmonton.
I’ve never ridden a train, other than the NYC subway. 
Maybe one of these days, we can get enough time off to ride Amtrak out to L. A. or something. Although the horror stories I’ve heard about Amtrak make me hesitate.
Thirty-six hours riding the Coast Starlight from Seattle to L.A., in 2005, and it’s not without reason that it’s also known as the Starlate. Still, from the perspective of one who was traveling as a tourist, it was an enjoyable experience. I’d do it again.
DC to Chicago and back, twice; about 24 hours each way.
Might do DC to Austin via Chicago and back in the future.
We took Amtrak’s California Zephyr on our honeymoon; we were on the train for about 16 hours – boarded in Chicago at around 4pm, arrived in Denver at around 10am the following morning.
We were able to get a sleeping room, but it was an “economy” sleeper, with an upper and lower single bed. The upper bunk was very claustrophobic, and you kept feeling as though you were going to pitch out of it as the train rocked; we split the night in the upper bunk, but neither of us were able to get any sleep up there.
We’d wanted to take the train as far as San Francisco, but weren’t able to get a sleeping room of any kind for the second leg (Denver-SF), so we got off in Denver, went up to Estes Park for a day, then flew to SF, but missed all the cool scenery which we would have seen going through the mountains west of Denver.
The food in the dining car was very good, at least. 
I did Cape Town to Johannesburg (and back) twice over a decade ago. That’s about 26 hours one way. It was a really pleasant experience. By no means luxurious, but they got the basics right and it was extremely comfortable. I remember liking the food too. I doubt it was fancy but it was tasty comfort food. Spectacular views!
In more recent years I’ve done San Francisco to Seattle twice. Also pleasant, but the sleeper cars were much more cramped. Still, its a really relaxing way to travel, and I sleep like a baby.
I should clarify that we were only late by a couple of hours or so. The scheduled 36-hour duration of the run is largely due to rail traffic volume into and out of L.A., and unusually tortuous stretches of track in the mountain areas.
Longest for me is a little over eight hours, Kuala Lumpur to Singapore. Even in First Class it was… unpretentious but it was an extremely relaxing trip and one I thoroughly enjoyed.
It’s also worth some cred among my travelling friends as I’m the only one who’s arrived in Singapore by train ([Seinfeld]“But it’s an island, Jerry!”[/Seinfeld]. :D)
Karachi to Peshawar on the Khyber Mail a couple of times. Famous train for being late but we were only a couple hours late on the outbound journey and right on time on the return. Distance of just over a thousand miles. I see the scheduled time is the same today as it was 30 years ago when I took it, about 32 hours.
We had a first class (non airconditioned) sleeper that my brother and I shared with a father and son all the way. The accommodations were fairly basic, but I remember the food being great.
I did ride a train second class, no reserved seating for 16 hours once as a teenager, with my tutor. That was pretty bad. We were packed in like sardines. The ticket collectors took bribes, so people would get on without tickets, and then pay off each ticket collector that came around. Every two to four hours. This might be more expensive than buying a ticket, but they only sold as many tickets as there were seats. The rest, well over half the passengers were ticketless. Prayer time was fun, though. Think about everyone in a crowded bar needing to roll out a rug at the same time. Everyone was rather good humored about it.
22 hours from Boston to Pittsburgh. This doesn’t seem too bad compared to other posts in this thread, but there was a nine hour layover in Philadelphia. Again, this still doesn’t seem to be so bad (time to explore the city!), but I had my sixteen month old with me. She was the reason I didn’t fly; apparently her ears didn’t clear when we were in the air, so she screamed her head off for most of the flight!
Am a UK citizen and resident; my own land, cramped little neighbouring continent, and my few ventures to other continents, have not allowed me to rival the North American / Russian / Australian / African experiences of various posters. My longest ever single-train journey has got to have been Shenyang to Chengde, China: about 14 hours – the train got delayed, and ended up taking a couple of hours longer than scheduled time. Exceeding thus by an hour or two, my next-longest: several runs between Hook of Holland and Poznan (Poland).
Nine hours, Chicago to MSP. We were delayed an hour or so by hitting some debris on the tracks, uncoupling some hose or other. No big deal.
About 14-15 hours. It was supposed to be 8, but some asshole jumped in front of the Amtrak and it took awhile to sedated, remove, and replace the traumatized driver, pick up all the pieces off the track, hose off the pieces that couldn’t be picked up, and clean the gore off the train.