On most railroads, when a regularly scheduled train was sold out as far as space was concerned, the overflow would be handled in a train running behind the original consist by a certain time interval. This train would keep the same schedule as the original except a set interval later. The original consist would display green flags by day and green marker lights by night on the lead locomotive to indicate a second (or more) section(s) would be following. This was required of each section, except the last. During the heavy travel periods in the heyday of train travel, many premier trains would run in as many as five sections, somewhat like streetcars. The last section running on a given schedule displayed no flags or markers, indicating it was the final train of that scheduled series. Extra trains were just that--trains operated in addition to regular schedules. Most extras were troop trains, camp trains, Boy Scout extras, baseball, football, race track specials, and fan trips--in other words, chartered moves. They didn't normally adhere to a set schedule although, they were first class trains. Extras were identified by white flags by day and white markers by night on the lead locomotive.
As far as baggage cars, in many cases they were the only thing that kept a lot of trains in the timetables. On many railroads, once great name trains went to oblivion as a worn out coach trailing a string of head end cars. It was the mail and express that helped make up what was lost moving people. Even some commuter trains carried a head end car transporting bundles of daily newspapers to the suburbs. If there wasn't enough papers to warrant a separate car, usually the first vestibule of the first car behind the power became an impromptu express car with bundles of newspapers. Then, Budd came along withe RDC-2 and RDC-3.
Personal experience: Left South Station in Boston on one night in December of 1964, maybe a week or ten days prior to Christmas, on New Haven train 185, the New Yorker bound for Penn Station, New York. Three FL-9s, 25 head end cars, two occupied 8600 coaches, and one deadhead idling RDC-1. No question as to what was paying the bills that night. It was more freight train than a passenger move!
A sight you don't see now: Many a time we'd pull into a smaller station and, there would be a hearse from a local funeral parlor backed up to the platform with the rear door open to receive someone coming home for the final time. Those stops were always made with a little extra care, simply out of respect. Now UPS handles human remains, instead of Railway Express Agency. A vignette for a model railroad? Occasionally?