Millions of Russians and Chinese, and several thousand Western tourists, travel the Trans-Siberian every year.
But there's a little-known fourth prong, too, the BAM, or Baikalo-Amurskaya Magistral (mainline), a single-track line that struggles northeast towards the central reaches of Siberia through desolate, uninhabited mountains and tundra to end on the Russian coast close to Sakhalin Island, almost 9000km east of Moscow and 1100km to the north of Vladivostok.
Largely a freight line, it has few Russian passengers let alone any tourists.
Judi and I, who have traversed and enjoyed the other three routes, rode the BAM in late (northern) winter-early spring this year.
It took 10 straight days of travel (although we cheated and "rested" after eight days for one night in a remote Siberian hotel) and two changes of train before we reached Vladivostok and could head on to China.
We shared our second-class, four-berth compartments at times with a variety of new and changing companions, all Russian.
None was in for the long haul like us; they were locals making short trips, railway staff, soldiers or other workers being transferred from one town or city to another, "engineers" moving on remote-area shifts.
It would be fully two weeks after leaving Moscow before we saw other Western tourists, in Harbin, China.
Ironically, they were a couple from Christchurch.
The BAM's main purpose is freight, not passengers.
It provides access to central Siberia's vast mineral-rich basins and forests and allows for the opening-up of previously virgin lands.
Oil, gas, coal and timber are the main riches, but there are stories of gold, diamonds and other minerals, even of huge deposits of uranium.
First mooted in the 1880s, construction of the BAM eventually began in the 1930s, helped later with Japanese and German prisoner-of-war labour.
It languished with Stalin's death, resuming only in 1974 when labelled "Hero Project of the Century" and the youth of the Soviet Union were urged to rally to the challenge.
It took more than 100,000 workers and almost 20 years of hard work in a terrible climate and through difficult terrain - swamps, seven mountain ranges, hundreds of rivers, vast swathes of permafrost - to complete the task.
The financial cost is said by the Lonely Planet travel guide to have been $US25 billion (the original Trans-Siberian line to Vladivostok, built in the 1890s, was estimated, in comparable terms, at $US500 million).
The BAM was officially opened in 1991, but the last and longest tunnel, at 15.34km, was not completed until 2003.
It has only been in the past few years that non-Russians have been permitted to use the few passenger trains on the line.
Our trip began in Moscow's cavernous Yaroslavl Station, reached by taxi through cloying midday traffic.
We had 200 teabags and six bottles of cheap French wine (a combination purchase that opened the eyes of the young girl in the tiny Moscow corner store), a dozen or so freeze-dried meals, packets of potato and gravy mix, several tins of salmon and meat, cheese slices and jam, the food all brought from home, plus two suitcases.
Even so, we were travelling light compared with many of our companions to come.
Train No 76 pulled out at 1.11pm, dead on time.
All Russian trains depart and arrive on time: the timetables are "managed" so they do, resulting in many unscheduled halts just before nominated stations and slower travel than could otherwise be achieved.
We had booked both lower berths in our compartment, thinking that would give us more chance of having it to ourselves, but the plan only worked sometimes.
We began our sharing experience at Moscow with a traditional Russian babushka, or grandmother, and her 7-year-old great-granddaughter.
They departed just seven hours later at Nizhny Novgorod (or Gorky, as it used to be called).
Other "guests" came and went during the next 10 days, none staying more than two nights, and some proving most delightful travelling companions.
We shared food, cups of tea (frequent cups of tea, Judi being a tea nut) and the occasional drink with many.
Russians, once the ice is broken, are most hospitable people, but conversations were generally stilted.
Use of a world map helped in showing where we came from, even though New Zealand was an unknown country to most.
Much of our time was spent reading or window-watching.
It was soporific to stare hour after hour at unchanging scenery of trees and grass and grass and trees through grimy double-glazed windows; we had plenty of short naps.
But meals, tea with the hot water always on tap at the carriage samovar, station stops and regular visits from the old dear selling pirozhnae (pastries) from the refreshment car and then early-evening wine, cheese and biscuits kept any form of monotony at bay.
Our route followed that of all Trans-Siberian trains for four days.
It was early spring; birch trees were coming into leaf and occasional tulips and daffodils coloured backyards.
At Krasnoyarsk, a concrete jungle with 900,000 citizens about 4000km from Moscow, the frost was heavy and snow still lay on the ground.
And as we turned north off the main line on to the BAM itself at Tayshet soon after, winter began to reappear.
Next morning, we gazed down on the Lena River, dotted with large ice floes and deep patches of snow on its banks.
Deciduous trees here still had no leaves. This was more like the Siberia we had been expecting.
The countryside soon began to resemble the Southern Alps or the Canadian Rockies.
Mountain after snow-clad mountain closed in on the winding, climbing, single-track line.
Huge rivers rushed beside, ice floes larger and larger. River ice still clinging to the banks was two metres and more thick. Swamps, ponds and lakes were sheets of ice.
We passed over the top of Lake Baikal, the world's largest body of fresh water. It was still frozen from shore to far-off shore.
Then came the 15.34km Severomuysk tunnel. It took 15 minutes to traverse.
Day six was desolation itself.
The train, now diesel-powered, hardly stopped, with legs of five hours, seven hours and seven hours through unrelentingly bitter countryside; nothing to pause for here save the odd timber-mill camp.
Eventually, we reached Tynda, population 39,000, headquarters town for the BAM, and destination for Train No 76.
We had 50 minutes to get off, find the platform for Train No 964, then find carriage 5, berths 21 and 23.
Easier said than done, given language difficulties, heavy cases, mountainous stairways and six different platforms (every station in Russia seems a mass of platforms, tracks, engines, wagons and people).
And when we did find Train No 964, there was no carriage 5.
Carriages 1, 2, 3, 4, 6 and 7, yes, but no 5.
With seven minutes before departure and no-one else speaking English, we were a little worried until an unsmiling guard was persuaded to read our tickets.
He assessed the situation and placed us in carriage 6, berths 21 and 23, and we continued our journey deep into Siberia.
At whistle-stop Fevralsk, some 670km and 16 hours on, all passengers seemed to desert the train.
Nobody but us and two provodnitsas (attendants) were left in our 36-berth carriage, and the whole train was reduced to four carriages.
We celebrated our solitude with tinned salmon, cheese slices and cold Efes Pilsener beer (60 roubles, or $NZ3 a large can), plus a can of Pringles chips - beer and chips bought from a small canteen aboard the train.
On day eight, after a breakfast of chocolate and hot tea, our train pulled into Komsomolsk-na-Amure station, and we gratefully disembarked into a grubby, graceless city of 300,000 souls, large aircraft factory, steelworks and shipbuilding yards on the river for a much-anticipated 24-hour stopover.
First up at a local hotel were showers and laundry - Russian carriages each have two grotty toilets with a cold-water handbasin, facilities you'd rather forget - followed by a decent stretch of the legs.
Shops were well-stocked if rather old-fashioned.
Food was plentiful and good; money machines and internet facilities were available; casinos were popular with the young.
Cleaner and refreshed, we caught Train No 351 heading due south to Vladivostok next morning.
The BAM line itself continues for 500km east from Komsomolsk to Vanino and Sovetskaya Gavan, the last stop right on Russia's Pacific coast.
From Vanino, ferries can be caught in summer to Sakhalin Island, weather permitting.
Spring began to reappear as the day and the journey progressed.
Leaves unfurled on silver birches, magenta flowers popped up, fresh grass emerged from the mud; we even saw some frisky baby goats.
At 9.58am the following day, dead on time, the train pulled into Vladivostok. Our 10-day, 9598km saga on the BAM was over.
And we still had 12 teabags left.
