Fully underground subway systems are more resilient
The Metrorail heavy rail transit system is a mix of underground and above-ground sections. In the worst winter weather conditions, in the past they have hunkered down and only run the part of the system that is fully underground, within DC (although some sections in the suburbs are also underground, primarily the Orange Line in Arlington County, Virginia).
The last couple storms, including the one this past weekend, they have shut down the entire system for a time, not even running the underground section, because of fears that potential power outages could strand trains and riders.
Greater Greater Washington.
I am not criticizing WMATA for shutting down, even though it turned out we didn't have widespread power outages (likely because the snow was soft and powdery).
You can try to keep the system running for as long as possible but on the backside, it means more downtime as the system works through the recovery period.
With an anticipatory shutdown, there is more uptime overall.
Still, with the benefit of hindsight, it's unfortunate that more parts of the system weren't built underground.
For example, if the section of the Red Line from Union Station to Silver Spring (between Silver Spring and Wheaton actually) had been built underground, then the entire east leg of the Red Line could operate during the worst of winter storms.
Similarly, the new Silver Line segment in Fairfax County was built above-ground and in aerial sections for great lengths. In the snowstorm--at least 20 inches in most places, and more than 2 feet in others--It turned out to be the most vulnerable section of the system and will be the last to be fully operational as the system reopens.
Too bad that winter-time resilience wasn't one of the factors considered in the debate over whether or not to build the Silver Line underground ("Rail Tunnel Debate Raises Larger Issue," Washington Post, 2006). It would have cost much more, true, but would have been able to operate in adverse conditions.
STM in Montreal is the best example in North America of building a heavy rail system completely underground, which they did in part because the city experiences some of the worst winter weather conditions on the continent. As a result, the Montreal heavy rail system is much more resilient than heavy rail systems like DC's.
-- Using public transit in winter | Société de transport de Montréal
Ironically, because the system was built entirely underground and train cars are not exposed to weather--rain, snow, or sun--the cars were not built to be weatherproof. (That means no air conditioning either.)
Two lessons for transit system design engineering derived from hindsight concerning the Metrorail system. (1) Fully underground systems are more resilient in adverse winter weather conditions. (2) Two track systems--one in each direction--lack redundancy, at least one additional track would allow for express service and more efficient operations when tracks or trains are not functioning.