Cheongnyangni is a transfer hub on the city's east side and the departure point for trains toward Gangneung and the east coast, so plenty of travelers pass through with bags. The catch here is simple: the locker bank is small.
Coin lockers at Cheongnyangni Station
Cheongnyangni Station (Line 1) has T-Locker coin lockers at one spot, about 17 units, by Exit 4 (under the stairs). That is not many, so on a busy day check what is free first on the Cheongnyangni Station page before you haul a case down.
Prices are the standard Seoul rate — about ₩2,200 (small) to ₩4,400 (large) for four hours. English screen, card, no ID.
Heading to the east coast?
Cheongnyangni is where KTX and ITX trains to Gangneung and Jeongdongjin depart, alongside the Gyeongui–Jungang line. If you are storing a bag before a day trip, do it at the station you leave from and pick it up on the way back.
When they are full
With only ~17 units, this bank fills fast. If it is full, try a station with a bigger bank such as Dongdaemun a few stops away, or see the full guide, which also covers the staffed desks.
Sources: prices, sizes and hours from the Seoul Metropolitan Government (T-Locker); spots, counts and availability from Seoul Open Data.