With the polar vortex deep freeze setting in, Twitter user @sumergocognito wants to know when it makes more sense to take the Blue Line than the Yellow.
— sumergocognito (@sumergocognito) January 3, 2018
The Blue and Yellow Lines share tracks and stations between King Street and Pentagon, and the stations from National Airport to the south are all outdoor, which means that in cold weather, it can be very unpleasant to wait for trains.
If you're headed directly for central Washington or the Mid-City, you probably want to wait on the Yellow Line. But if it's a long time away, that wait can seem cold and grating. If a Blue Line train is coming first, it might be tempting to take that one to L'Enfant and transfer.
If time is your primary motivator, though, that's not the best solution. It takes 12 minutes for a Yellow Line train to go from National Airport to L'Enfant Plaza. But it takes 30 minutes for a Blue Line train to do the same. That means that unless the next Yellow Line train is at least 18 minutes away, taking the Blue Line will make you miss that next Yellow Line train.
But don't despair, because there is a way to beat the long, cold wait. Take the first train that comes, and if it's not on the line you want, transfer at Crystal City, Pentagon City, or Pentagon, which are all underground stations.