Numbers in the 600-series were used at that time within the TTC for rapid transit routes (i.e., subways and the Scarborough RT) rather than single-digit numbers as is the case today, and the Harbourfront LRT was given the number 601 to indicate that it was different from other streetcar routes. The 1990 "TTC Ride Guide" indicated the route with its own colour like a subway line rather than as a streetcar route. Since the route is not grade-separated, this was later felt to be misleading and it was treated like other streetcar lines, taking the number 510.