The Tribune has a good report with further details and photos on the Brown Line mishap this morning, including video and photos.
The CTA is not calling the incident a "derailment" because "all of the wheels stayed on the track":
[CTA spokesperson Noelle] Gaffney said the eight-car train had just left the Belmont station
and was headed to the Southport station when the front part of the train
switched onto the westbound tracks on the Brown line while the rest of
the train's wheels remained on the northbound tracks.
"The first part of the train went on the correct track, but the middle part stayed on the northbound track," Gaffney said.