Recently, I blogged about KT’s plans to install wireless Internet service in the subway trains of Seoul, South Korea. Today I report on a related update: WiMAX day says that SK Telecom yesterday filed document with the Korean stock exchange announcing the company’s intention to invest $125.8 million to expand WiBro services in the country.

(In case you’re note familiar, WiBro is “the local brand name for mobile WiMAX in Korea,” WiMAX Day explains.)

The connection to the subway project, by the way, is that SK and KT are jointly working to install wireless Internet in the trains.

SK said its $125.8 million investment will be used to expand WiBro service in 20 Korean cities, as well as subway stations in Seoul, WiMAX Day reports.

According to WiMAX Day, SK Telecom is facing something of an uphill battle succeeding in its wireless initiative. The company launched WiBro service in June, and “has been struggling to gain subscribers and expand the network.”

The struggle, WiMAX day says, is due to two main factors: limited covers of the services in Seoul, and lack of available WiBro handsets.

Partnerships seem to be part of SK’s plan to roll out WiBro in Korea. In November, the company teamed up with Hanaro Telecom to help bring wireless to more areas. The rumor mill has it that the partnership could become a permanent marriage; it’s possible the two companies will merge at some point in the future.

Only time will tell how successful WiBro is in Korea.