Metrorail entrances often have 2-3 escalators. For those with two, it makes sense for them to run one up and one down' nowever, should one not be operational, have the other set to take customers up the escalator. This helps elderly and infirm who typically find it easier to walk down than up. For those with three escalators, run at least one going up. During rush hour, run two in the direction from which most passengers either enter or exit.
This is really a no brainer, and is only necessary to spell out because station managers don't pay attention to when people need the escalators running up or down. So, its basically a training matter. A Metro Center station mngr. once told me they always run 2 escalators up - even during the PM rush just for safety exit purposes. However, there is no consistency and thus no reason to believe this is actually a rule at stations. On 6/22 for the AM rush at the 13th st. exit of Metro Center there was one escalator working - going down. If it actually is a rule, its not well considered.