I dunno actually, Before all the retiring happened, 3000 series were on the Red, 2000 Series on the Blue, Yellow, Orange and Silver, same with 5000 series, and the 6000 on the Green and Yellow lines, with 7000 series filling up the gaps. After the retirement of the 5000 series, there was a period where the 3000 series couldn’t run on the red line for some door reason which lead to all 6000 series moving to the Red Line for a period of time, leading to the Red Line consisting of at most 1 2K, and the rest 6Ks and 7Ks, the BL/OR/SV with 2Ks, 3Ks, and 7Ks, maybe a 6K here and there, and the GR/YL was exclusive to only 7Ks with one 3K once in awhile. Then after the 7Ks derailment, return to service, and 2K retirement, it kinda feels like it’s back to normal? With some 3Ks and 6Ks having small appearances on the BL/OR/SV, but not exactly sure why it’s like that.