So I had a Grand Central to Harlem 125th street ticket that I hadn't used and thought I would use it tonight and pay the difference. My understanding was that iff I already purchased a ticket and just need a step-up ticket, it's only the difference between the zones. In this case, I had an off-peak $6.00 ticket, and getting to Spuyten Duyvil is $6.75, making the difference $1.00 when rounded up.
I tell the ticket collector what I need, give him the ticket and $1.00 difference. He tells me I have to pay $7.00 for the difference. I was incensed. Why is MNRR charging a $6.25 "penalty" for a step-up ticket? I was under the impression that you can't buy step-up tickets anywhere else but on the train. The MVMs don't sell them to my knowledge, and I believe the ticket booth at Grand Central was closed, so how else would I get a step-up ticket? There's also nothing posted on the 
website about this which makes it even worse. I refused to pay the penalty and got off and took the express bus home instead.