This is only an observation, not a 100% generalization. In these winter months of early 2019, most diesel trains passing through Wantagh have a locomotive and four cars. I presume that the rush hour trains don't fit the same pattern, as Train 2735 for years has been six cars shoved west by a locomotive on Track 2. Even Oyster Bay seems to have the locomotive + 4 coaches standard. The practice certainly cuts down on labor costs of adding or removing coaches. Maybe it cuts down on problems with frozen connections. Summers will see more variation.