Additional elevators are being added at DTX in several places - the official name is Downtown Crossing Vertical Upgrade, but good info is hard to find. I believe the work you're seeing is the addition of a pair from the NB OL (Summer Street concourse) to NB RL platforms inside fare control (so that transferring handicapped passengers don't have to make their way down their whole concourse and re-enter fare control). A little bit of that is described
here and
here.
If I recall, there will also be elevators from the concourse to the SB RL platform, and also elevators from the Winter Street side to the RL. But don't quote me on that.