The Express F train is happening, but maybe it shouldn’t.

Today the NY Daily News has reported that, come the fall of 2017, the F train will regularly run express in Brooklyn for the first time since 1987. Normally that would be something to cheer — transit expansion in New York City is rarely a bad idea. However, there’s a key detail lurking the article that changes the calculus considerably: there will be no new trains. This means that the new express service will be taking trains away from the local service. It’s a transit reassignment, not expansion.

So, why is that such a problem? I looked at the data behind this back in August and the key takeaway is that the local F train stations carry a lot of passengers, particularly when you get closer to Manhattan — the block of five stations between Bergen St and 7th Avenue (four of which are local) carry nearly as many passengers as the remaining 12:

That’s not all, either. This express service will skip 4th Avenue — 9th Street, a key transfer point allowing passengers on the R train a transfer to the F without joining the crush of A/C passengers at Jay St — Metrotech.

The report itself suggests a 50/50 split of express to local trains, which would result in lower average travel times — while local passengers would lose 1.3 minutes, those at express stations would gain 3.4 minutes. However, this is based on train timetables alone, and as anyone that regularly commutes via the Bergen St stop knows, some peak trains are so full that they require passengers waiting for another train.

So where does this leave us? It certainly seems like a valid experiment, but a 50% reduction in train frequency at both Bergen St and Carroll St is not likely to be popular.

An earlier version of this post was written before I’d seen the full report, I’ve since updated it with the express/local split and average time savings.