The four of us in this tiny compartment know why. Nikolai, one of our company, has a supply of raw fish wrapped in newspaper under his bed.
Half an hour ago, not for the first time, he dug out a couple, gutted them on a cloth on his bed and, with the air of a cordon bleu chef preparing a particularly special dish, chopped them up for us to eat.
He is a genial, middle-aged man, with the fixed smile of a cat and a shiny blue shell suit that would stand out on the Strip in Las Vegas. For the last two days, as our train sidled slowly across the contours of Russia's stomach, he has regaled us with stories about his life as a factory official in Irkutsk in eastern Siberia. But the fish is a problem.
We have heard about how he once shot, and ate, a bear in the forest, and later went hunting for wild boar. He has described fishing through the ice that each winter covers nearby Lake Baikal to such depths that, during the 1904 war with Japan, the Russians laid a railway across its frozen girth and shunted equipment back and forth.
The track hooks around its southern shores, so several days ago we saw the lake for ourselves, a vast grey-blue sheen, the size of Belgium. The guide book overflowed with statistics: it's 400 miles long (large enough to have storms which never reach its shores), 50 million years old (ancient enough for 1,500 endemic species), more than a mile deep, repository of a fifth of the world's fresh water supply and its own species of seal, the nerpa.
But these figures, and the lake's hazy beauty, have been jogged to the back drawers of the memory by the slow lollop of the last 1,400 miles. Only one detail looms large: it was there, during a brief stop in a small settlement on the lake's shore, that Nikolai bought his food supplies. Baikal is the only place on the planet where you can find the golomyanka fish, which dissolves into an oily blob when brought to the surface. Unfortunately, Nikolai bought "omul". They have remained intact.
There is no point in complaining about the stench to Marina, the bored-looking peroxide-haired carriage attendant. The windows of our German-made carriage cannot be opened, as it is supposedly air- conditioned. Her principal task seems to be to hoover the carriage every 24 hours or so, usually when we are nodding off. Nor is there much evidence that she is on the ball; she has been spotted wandering the corridors in a long crimson dressing gown, well after day-break, with her locks in curlers.
But, then, no-one on the train seems to know the time. Small knots of people gather in the corridor to study a timetable on the wall, trying to relocate themselves in the surreal vacuum that has evolved since the train set off.
There is a clock showing Moscow time, but it doesn't help much; we are running late, and most of the passengers have started from a different point on the seven time zones through which the train passes on its 5778- mile journey from Moscow, across the Urals and Siberia, to Vladivostok and the Sea of Japan - the longest continuous rail journey in the world. Very few people seem to be travelling the whole way. For most, the calculations become too complicated to be worth the bother.
Outside, the "taiga", the endless forests of silver firs, cedars, birches and pines, has given way to the softer, flatter landscape of the far, Far East. Every now and then we pass a clutter of wooden bungalows, their mud lanes littered with the detritus of Soviet farming equipment.
It looks as old as the railway we are riding, which was inaugurated in 1891, under Tsar Alexander III. The faces of the few residents grower wider, testimony to the proximity of Mongolia and China. But neither these, nor the grey sky overhead, offer many clues. Nikolai is convinced it is Tuesday; I know it is Wednesday.
As we argue, Nikolai pours from a vodka bottle on the table. When I boarded in Irkutsk two days ago, I believed this warm, acidy liquid to be the real thing, until I discovered him topping it up from an unmarked plastic container in his luggage. "Medical spirit," he explained. "Now, I have got this neighbour, who loves hunting ..."
There is, of course, a great deal of drinking. The other day a group of Russian army officers, their bellies awash with booze, held a press- up contest with a young British civil servant during a station stop. The Briton, a tourist en route to Japan, later told me he had "lost" two days of the seven and a half day journey from Moscow.
But in an environment in which everyone shares everything, drinks are as hard to refuse as food. In the hope of avoiding another meal of fish (or sausage, of which Nikolai has an equally large, equally pungent, stock), I have produced a pot of Skippy peanut butter. Neither Nikolai, nor the two young female students who share our space, seem enthusiastic, but they sample it politely.
Russians, long used to cramped apartments, are good at this kind of collective living. Our four-berth second class compartment is only six feet wide and seven feet deep. But my companions move easily among themselves as if they were somewhere four times as large. When one of us is making up a bed, or changing clothes, the others automatically slip into the corridor, without exchanging a word. I am the only clumsy one. A couple of hours ago, a large pepperoni sausage fell from my bunk onto the head of one of the women.
Yet there are few places to which to escape these kinds of embarrassments. You can perch on the small, fold-down plastic seat in the corridor, although not without feeling foolish. You can also retreat to the restaurant car, although very few of the mostly Russian passengers on this train go there, not least because, for many, a plate of sinewy chicken and a beer costs the equivalent of a day's pay.
One visit was enough to discover that serving food was not high among the staff's priorities, although they were keen to sell the gas masks which they claimed to have been issued in the (unlikely) event of a gas attack by Chechen terrorists. The rate was six dollars, a strikingly better bargain than the $20 that one carriage attendant wanted to charge a tourist for his metal tea-cup holder, or the $10 he wanted for attaching a shower nozzle to the tap in the grubby wagon lavatory.
The restaurant car is the fiefdom of Mikhail and Sasha who, when they are not selling huge quantities of liquor to villagers at each station, wait for foreigners like me to wander in to relieve the boredom.
"Are you English?" demanded Mikhail. "What happened to your football team?" he said, before, somewhat contradictorily, reenacting Gazza's goal against Scotland in the space between the empty tables.
"Why is England good at nothing these days? You haven't got a number- one skier, tennis player, skater, ice-hockey player, boxer. Name one!"
There was a pause, and his mind turned anew to business. "Would you like me to find you a woman?" As the Russians say, "Para iti": time to get back to Nikolai's fish.