Some of the K27's had superheaters added. The non super-heated models were probably the first to be retired as newer K's came along. As NG traffic declined, the older '27s simply became surplus power. The ones that survived were ones that replaced the C's as yard/branchline engines. It's also my understanding that crews liked the K28's. The San Juan and Silverton branch kept the 470's alive -- remember that nothing larger than a K-28 had been in Silverton till 1981. Otherwise 473/476/478 might have ended up in Alaska with the rest.