It isn't the only reason, but it is part of it. If you decompose into 60 foot equivalents, we have fewer cars now than we had pre-2010. There are roughly 130 spare R68s, amounting to ~16 spare 8 car trains. With inspections and breakdowns, that doesn't leave much room for extra service. Theoretically, they could throw some R46s on the , but again, those are basically reserve cars for the .