retired engines in denver?

I happened to be in denver on a trip and took a ride on the light rail system and we drove past a UP freight yard, and saw large number of old engines and a number of switch engines as well. Are they all retired, because a number of them still looked in running condition. And why so many just sitting there? Any waiting for repair?

Most likely they are being stored till they are needed again. The economy is down and there are engines stored all over the system.