South-west of Takasaki the Kantō plain runs out and the mountains begin, and this stop lies right on that edge, between Annaka and Tomioka. Look west and the strangest skyline in Gunma comes into view: Mount Myōgi, a crown of jagged volcanic spires that looks as if someone took a rasp to an ordinary mountain. It is one of the three famous peaks of the old Jōmō province, and the road that skirts its flanks is a favourite with local riders for its tight, wooded bends.
Tomioka gave Japan its modern silk industry. The silk mill, opened in 1872 with French machinery and French engineers, was the government's showpiece of industrialisation, and its long brick buildings, now a World Heritage site, still stand in the town centre. Silk once paid for much of Japan's modernisation, and mulberry fields covered these hills.
Annaka is the gateway to the old Usui pass towards Karuizawa. At Yokokawa, where the railway once needed extra locomotives to climb the gradient, a railway park preserves old engines, and further up the valley the four-arched brick bridge of Usui, the Megane-bashi, stands in the forest like a Roman aqueduct. Yokokawa station is also famous for its boxed lunch of rice and chicken served in a small clay pot, which travellers have been buying for generations.
For a swim, keep expectations modest. The Usui and Kabura rivers are rocky, fast and cold, good for cooling feet on a hot afternoon but not for real swimming. Gunma's summer heat is fierce, so the shade of the riverbanks may be the best you get.
Fuel is easy in Annaka, Tomioka and Takasaki, and convenience stores are everywhere on the main roads. The roads into the mountains, though, have long stretches with nothing, so leave the plain with a full tank.