Missing Tank Cars?

I have read every one of these posts and find them fascinating. However, I am surprised to NOT see a certain eastbound train consist. The consist would be 2 locomotives, 11 loads, 1 empty. This would represent a Cumbres turn of 11 loaded tank cars and a caboose.
I have (somewhat) assumed that oil was on the move from Chama to Alamosa with a high degree of frequency. The assumption is largely validated by numerous pictures of trains in this time period showing tank cars. Some tank car loads went westbound (road oil and some gasoline) but the bulk of the business was running empty tank cars from Alamosa to Chama and returning with loads of crude oil to the refinery at Alamosa.
There are many pictures of "tank trains" going up the west side of Cumbres Pass. The consists nearly always have a 480 or 490 on the point, 11 tank cars, another 480 or 490 pushing, followed by a caboose. This calculates about right, tonnage wise, for two locomotives.
This post here comes close with 483 and 487 on a train but looks like 2 loads and 1 empty too many. (A similar situation occurred on July 17, 1957.) I would imagine a few loaded tank cars snuck out of town on some of the 3 engine freights that have been leaving Chama, however, sometimes I wonder when I look at the consist of those trains (loads vs. empties vs. tonnage vs. number of cars).
Anyone have any ideas why the tanks cars aren't showing up in "unit" trains? Inquiring minds want to know!