In Tanzania, if the train leaves at ten in the evening, you'd better check whether that's Swahili time. If it is, it could mean 4pm and you'd be very late. "Swahili time" starts the day at zero hours when the sun rises faithfully at 6am. Midday is six o'clock and 4pm is ten. Tanzanian households commonly set their clocks to Swahili time.

As it was, the Kilimanjaro Express (going nowhere near Kilimanjaro) finally left Dar es Salaam punctually at 4pm. It chugged for 40 minutes past waving children and dusty games of soccer, then stopped at the rubbish-strewn outskirts of the city and, to the children's amusement, reversed back to Dar. It left again, with a new engine, at 10pm. So it wouldn't have mattered either way.

At least it meant we'd see more of the changing scenery of Africa on the 19-hour journey to Mbeya, near the borders of Zambia and Malawi. There, our tour group was to climb a volcano with a crater lake that harbours a mythical monster.

After dinner there followed a restless night in our four-berth sleeper, listening to the train wheels screeching. My Swahili phrase book - bought at the station - came in surprisingly handy with "there is a grinding noise".

Come daylight I decided to try the first-class bar, with its green velveteen sofas and tatty carpet. The countryside changed from dry savannah woodland to forested hills splashed with red erythrina blossoms, to villages of tin-roofed houses with people hoeing small vegetable plots.

The train didn't stop often, but it wasn't very "express" either, labouring up the hills as we climbed to nearly 2000m. When it did stop, traders plied hard-boiled eggs and raw vegetables to the third-class carriages, perhaps knowing that we first-class passengers had fish and rice served by white-shirted, bow-tied waiters. All we were offered were custard creams and chocolate.

By the afternoon, the landscape was parched: thorny trees, small cylindrical granaries roofed with a witch's hat of thatch, baobabs, a child herding scrawny goats through a dry riverbed. This was the Africa of charity posters. It made you thirsty just to see it.

Inside, the train was equally dry. Although the bar was fully stocked with cold Kilimanjaro beers and sachets of Konyagi - local gin - costing 20p, there was no water for tea or coffee. Nor to wash away the heat and dust. A locust blew in through the window and landed on the table next to my bottle of icy Tangawizi ginger beer as I sat listening to the barman's selection of Congolese soukous and George Michael. (In second class, there is no cosy bar, but the carriages have their own DJ: a train hostess dressed in a burgundy suit sits in a small "broadcasting room" playing an equally eclectic selection of tapes.) Eventually, after I'd watched a second sunset from the comfort of the sofas, the "Express" arrived at Mbeya. Remind me not to take the slow train.

Mbeya thrives on being a border town. It's a busy, dusty centre of commerce (street trading rather than showy high-rises). People come here to sell potatoes, onions, maize and tomatoes. Unlike Mbeya itself, where the river is a trickle in a deep gorge of sand-coloured earth, the nearby Poroto Mountains are high and lush, even in the dry season.

At the station, a lively, smiling young man approached. He was our guide, Niko Ntinda. Now thirty, he was educated only halfway through secondary school because his parents couldn't afford the fees. Despite (or perhaps because of) this, he is a determined man.

Recently, with help from a Dutch aid agency, he started an innovative tour guiding business with friends. Now their workers' cooperative rents its own office and contributes to agricultural projects for youth. Most customers are backpackers who pass through Mbeya en route to Malawi. Niko soon proved invaluable. A chubby, white-uniformed policeman directed our vehicle to the roadside, demanding a "fine". We avoided payment through Niko announcing that we were missionaries off to preach God's word. Forgive us Father.

Niko didn't think much of the newly re-elected President Mkapa's anti-corruption drive. "The traffic police are the worst," he said. At a later, similar incident, our bus driver extricated us by showing his party membership card and saying we were in Tanzania as guests of the ruling CCM party.

In the hills above Mbeya, along a smooth tarmac road, the countryside changes from oppressive dryness to wholesome greenery. At 2400m, the temperature has the pleasant coolness of an English summer. Lilac hydrangeas offset brown mud-brick houses. The grass is green. The trees actually seem alive - eucalyptus, peach, Silk oak. Among fields of maize are little plots of white daisies. "Pyrethrum," explained Niko. "It's used as an insecticide. Even a single flower in your house drives insects away."

We soon reached the volcano turn-off - a bumpy dirt road with a crude wooden barrier. Here visitors pay an entrance fee, supposedly to maintain the road and the local school. After some debate with a shifty-looking villager who looked more likely to spend the money on pombe (beer), we parked and began walking.

It is an hour's fairly tough hike up Ngosi volcano - although we had no sense of climbing any particular geological form, as we were surrounded by forest. There was one opening, into a thicket of bamboo which looked dead and bare, like a giant's stock of kindling, although its new shoots were ready to sprout with the rains. The bamboo is the haunt of black and white Colobus monkeys, which we inadvertently frightened away.

After a particularly steep section, the path opened onto a superb view. We were on the rim of a large crater, looking 200m down sheer tree-clad cliffs into the blues and greens of a vast lake, shaped like Africa.

Ngosi Crater Lake is magical and mysterious. No-one knows its depth nor what lives within. Local people believe the cobalt waters harbour a monster. "Nessie's cousin!" joked Niko. If the rains are poor, villagers climb the volcano and clamber down to the lake's edge to make a sacrifice. "A black hen or goat is killed, then burned on a fire," said Niko. The people eat the meat and the monster, supposedly satisfied with the smoke, rewards them with rain.

It had been an amazing day. I tried to impress Niko with help from my Swahili phrase book. But the book's expression for "I have enjoyed myself very much" turned out, to everyone's amusement, to be a phrase reserved for more saucy occasions. Swahili time. Swahili phrase books. You could find yourself in deep water.