Notorious TeamTron member, Henrik Has-Never-Been-Defeated-In-Old-School-Magic-In-Denmark Storm and I, tried to boil down a definition, but the closest we ever got was something along the lines of: A synergy between two or more cards, that is easily disruptable . I dont think that is very precise nor a very useful definition, because by this definition you will open a way too broad aspect of the game to being combo
I never thought I'd end up spending over $100 at a McDonald's in one trip
Its limited to finding instants and sorceries but does so at instant speed for just 1 mana, and sets your library up with exactly the card you need to draw for whatever your situation might be
10 Emrakul, The Promised End Emrakul, the Promised End is a 13/13 legendary Eldrazi from Eldritch Moon
Scheming Symmetry plays a role akin to that of Imperial Seal
Cards similar to Subway Train include Strixhaven Skycoach