If you want to be really technical, trains don't have numbers, schedules do. A schedule is an authority for a train to occupy and run on the main track. When an engine, or motor, is assigned to a schedule, it can use the main track and displays the assigned schedule number in its displays. It "indicates" to one and all which schedule it has been assigned, assuming the railroad uses indicator displays. It makes no difference what kind of equipment is indicating the schedule number (freight, passenger, etc.). Whichever locomotive is authorized to display "98" in its indicators actually is "schedule 98" (colloquially known as "train 98"), even thought it might be a lowly switch engine.

If the DS wants to run more than one train on a specific schedule authority, each of these is called a "section", and he will issue orders to all preceding sections to "display signals for sections following" -- those "green by flags and green lights by night" (the lights colloquially known as class lights). Each section will also display 1-87, 2-87, 3-87, , , , 17-87, as instructed, with the last section displaying no "number-", just "87" and no green signals for sections following.

A train turned out for (in a siding waiting for) train 87 --"meeting" 87-- will have to sit and wait for the section which isn't "displaying signals for sections following", no matter how long it takes for that last section to arrive the siding, before it can re-enter the main track and continue on its merry way. In this case, the train will have to wait for 18 trains, 17 displaying signals for sections following plus one displaying none (I have it on good authority this actually happened on the SP during WWII). Sections can't pass each other without taking certain precautions regarding their exchanging indicator displays etc. because when that waiting train sees "3-87" go by, called "checking 3-87", it can assume both 1-87 and 2-87 have already gone by. Remember, there was only telegraph; phones came later. And never mind "Marconies". You had to keep your eye peeled for what was going on.

By using indicator displays, it wasn't necessary to supply a train order to every inferior train listing each and every engine number of all superior opposing regular, superior trains and sections thereof. Some railroads did this, SP, SN; others didn't, WP, AT&SF. The "other rrs" had to send out a lot of orders identifying the engine numbers of each section and schedule. Which method is right and which is wrong? Irrelevant question.

The SP stopped using indicators, except on the Peninsula (and in a sense Caltrain still does), because with the first generation computers X, being a letter, couldn't be entered in a numeric field (the engine number field) of the SP's database. It was a similar thing when Ma Bell forced all digit dialling on us. No more "PEnnsylvania-6 5 thousand", although the meter of "736 5 oh oh oh" does work. The White House used to be NAtional-8 nnnn.