Turns out that Trail isn't that useful a counter to Skirmisher, if there's more than one Skirmisher in play. This happened to me in a recent game and really confused me at first
My opponent had several Skirmishers in play and bought another Skirmisher, forcing me to discard. One of the cards I discarded was a Trail, so I got to draw another card. But then the server forced me to discard again. It happened that I'd drawn another Trail, so I discarded that, but then I had to discard again, and this time I discarded an Estate. I couldn't figure out what was going on at first, and I thought maybe there was some glitch with the server, but then I realized what was going on
Each Skirmisher that was played had the "discard down to three" effect. Normally that doesn't matter, because once you've discarded to the first Skirmisher, the other Skirmishers can't do anything. But in this case, I had more than three cards after resolving the first Skirmisher, so then when the next Skirmisher was resolved, I had more than 3 cards in hand and thus had to discard again!