Tram 19 has been running between Delft station and the TU Delft campus for a few weeks, about twenty years after digging began.
The hold-up was physics: the route passes labs where magnetic fields and vibration would disturb sensitive measurements.
Emeritus professor Pieter Kruit, whose own lab sat on the route, raised the alarm when work on the Mekelweg started.
Two ways to cancel a magnetic field
Regio Haaglanden, now MRDH, and operator HTM told researchers to shield their equipment with Helmholtz coils, which Kruit found awkward and expensive, so he tackled the source instead.
Most of the current now runs through a thick cable under the rails, rising to the overhead line only at the tram itself.
It also reaches the tram from both directions, so the magnetic fields cancel out.
The rails sit in thick rubber so current cannot leak into the ground, and two other TU researchers added a concrete substructure to limit vibration.
TU Delft patented the design, now also used on the Uithoflijn in Utrecht and a tram line in Lund, Sweden.
Kruit says the patent has earned the university about half a million euros, and he calls the physics first-year material.
