“MH” Sign at Jarvis

[Chicago.trains]

I was at the Jarvis Red Line stop this evening and noticed a small trackside sign today. It was very dirty and seemed like it had been there for a while, about the same size as the car birthing markers, but much lower to the ground. All it says is “MH”. (Unfortunately I wasn’t able to grab a picture today.)

I haven’t come across such a sign before and searching around yielded no results. My curiosity has been piqued by it and I’m hoping maybe someone here knows.