Not sure about 60, but I believe something like 75s is theoretically possible. The issue that occurs when you approach the 90s barrier is that to maintain such high headways is that the margin for error in terms of any form of delay quickly approaches 0. If you're at a station and someone holds the doors open for even just a second, you can say goodbye 60s headways. However if we think of a space in the land of purely imaginary theoreticals, assuming you don't have any disruption with passengers holding doors or whatever, then in theory 60s is entirely possible. Back in the day interurbans all over this continent reached those headways on a regular basis. The Hollywood Tunnel in LA that was used by the Pacific Electric railway had 880 trains pass through it, which is around a train every 3.2 minutes per direction on average in a 24h period (IE, it was effectively 60s during peak periods), all done via the signalling power of Line of Sight. Something similar would occur on the NYC Subway where on the local tracks, subway trains would basically arrive back to back also thanks to the power of Line of Sight (trains would run slow enough that this was practical, also safety standards were far lower at that time).Is it even physically possible to run 60 second headways? I seem to recall reading some such discussion or other that said that 90 seconds is pretty much the ceiling on an operation like this. Can anyone confirm this?