Here’s one sign the subways have turned a corner since the disastrous days of summer 2017: A vandal has delayed hundreds of trains since March, yet New York City Transit in April still reached its best on-time performance rates since October 2013.

The scoundrel somehow acquired a master key that lets him access the empty conductor compartment in the back of trains (mostly on the 2 and 5 lines) and pull an emergency brake.

Now that the MTA has gone public with footage of the perp, his days of continued freedom should be few. Every straphanger will be on the lookout.

And despite this menace, on-time rates on the numbered lines hit a weekday rate of 85% last month, led by the No. 7 train, where performance rose by a third.

The 7 is one of just two lines that have seen a full signal upgrade. City Transit chief Andy Byford wants to speed up plans for systemwide upgrades under his Fast Forward program, which otherwise could take decades.

Hope that the MTA finds the tens of billions needed for that, because other lines have a long way to go: The lettered trains only managed an average 75% weekday on-time rate in April, with the A, B, D, F, N, W and R all below 70%.

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In all, one in five trains still runs five minutes or more behind schedule.

You can thank the MTA for what it calls “continued and dramatic improvement” — but millions of New Yorkers need the gains to continue.