A rail has to leave room for the wheel’s guide
Street-running tram rails are shaped differently from ordinary railway rails. Their top sits nearly level with the road so cars, bicycles, and pedestrians can cross it. Alongside the running surface, a narrow opening—the groove—provides clearance for the wheel’s inward-projecting flange, the raised rim that helps keep the wheel aligned.
That opening is not just a scratch in the pavement: it continues alongside the rail, forming a channel through which the flange travels. The wheel’s weight is carried on the rail’s broad top, while the flange works at the side to resist sideways movement. Closing the groove would obstruct the very part that helps guide the tram.
A tiny channel with a city-sized cleanup problem
Because the groove sits at road level, it is exposed to whatever the street sheds: grit, leaves, mud, and other debris. The channel is narrow enough that material can settle and compact there instead of being swept away with ease. The result is a small maintenance task built into a design that lets heavy rail vehicles share streets with ordinary traffic.
The tradeoff is easy to miss from the sidewalk: the rail’s low profile keeps the crossing smooth, but its guide channel also creates a sheltered strip where street dirt can accumulate. A tram needs that slot to follow its track; the city has to keep the slot clear.
