I used to say " on your left" on the rail trail but this seems to confuse most people who are not used to walking on the trail, especially during and after all this pandemic period.
I have decided that "passing behind you" called out from a bit further back results in the said families or groups of people usually splitting up on either side of the trail. so instead of trying to explain " no im passing on YOUR left" I have time to see where they move to and then just pick my line between them, or around whichever side they moved to.
I always try to say thanks or something polite even if what I really wanted to say was Get the F outta the way.