Meanwhile , NSB introduced gasoline railcars which stopped at the new flag stops .
Double tracking continued southwards , opening to Kolbotn on 15 December 1936 and to Ski on 14 May 1939 .
NSB introduced gasoline railcars which stopped at the new flag stops .
Double tracking continued southwards , opening to Kolbotn on 15 December 1936 and to Ski on 14 May 1939 .