两条平行轨道在一个狭窄的地方的交汇处；里面的铁轨横穿再平行前进然后分开以保证一辆火车总是在一条铁轨上运动。

• the convergence of two parallel tracks in a narrow place; the inner rails cross and run parallel and then diverge so a train remains on its own tracks at all times.