Mexican Train — engine conformance against online sources

This document records how the Warp 12 rules engine (RULES.md Sections I–V, the standard Mexican Train layer) was validated against the most widely cited online rule sets, and where Warp 12 makes a deliberate, documented choice when those sources disagree.

Mexican Train has no governing body and no single canonical ruleset — every published source differs on some details. This is not an appeal to authority; it is a “consensus reading” exercise. Where a rule is essentially universal, the engine matches it and a test enforces it. Where sources conflict, the engine picks a convention (usually tournament-style) and the divergence is called out here and in the test comments.

Sources consulted

Tag	Source	URL
[W]	Wikipedia, “Mexican Train” (Galt/Parsons 1994, Bauguess 2012 consensus)	https://en.wikipedia.org/wiki/Mexican_Train
[MoG]	Masters of Games	https://www.mastersofgames.com/rules/mexican-train-dominoes-rules.htm
[UBG]	UltraBoardGames (“Official Rules”)	https://www.ultraboardgames.com/mexican-train/game-rules.php
[GC]	The Game Cabinet (Galt-derived)	http://www.gamecabinet.com/rules/MexicanTrains.html
[DP]	domino-play.com	http://www.domino-play.com/Games/MexicanTrain.htm
[SC]	The Spruce Crafts	https://www.thesprucecrafts.com/mexican-train-dominoes-complete-rules-410911
[MTR]	mexicantrainrulesandstrategies.com (tournament PDF)	https://mexicantrainrulesandstrategies.com/mexican_train_rules.pdf

Automated verification

Two engine test suites keep this document honest:

mexican-train-conformance.spec.ts — one scripted scenario per rule below, each citing the source(s) it validates. Run yarn test:engine.
random-play-harness.spec.ts — a random-legal-move fuzzer that plays full games across every player count (2–8), both objectives, and all module / house-rule combinations, asserting tile conservation (the 91-tile double-twelve set is never created, lost, or duplicated) plus structural invariants after every action.

Conformance matrix — standard rules (RULES.md I–V)

Legend: Match = universal across sources and enforced by a test. Choice = sources disagree; Warp 12 adopts one convention (noted).

#	Rule	Sources	Warp 12	Status	Test
1	Double-twelve set = 91 unique tiles, 2–8 players	[W][MoG][UBG][SC]	91-tile set	Match	`Setup — the set is a 91-tile double-twelve`
2	Hand size 15 (2–4) / 12 (5–6)	[W][MoG][GC][DP]	15 / 12	Match	`Setup — hand sizes`
3	Hand size for 7–8 players	[MoG]=10, [GC]/University=11	10 default, host may pick 11	Configurable (`largeFleetHandSize` house rule)	`Setup — hand sizes` + `deals 11 tiles…`
4	Highest double set aside before the deal, opens the station	[W] (also 1994 Galt alt: dealt & holder opens)	Set aside, not dealt; starter rotates	Match (Wikipedia primary method)	`Setup — highest double set aside`
5	Spacedock steps down each round; double-blank is the last; 13 rounds	[W]	12-12 → 0-0, 13 rounds	Match	`Setup — Spacedock descends`
6	First player rotates each round	[W][GC]	Starter rotates clockwise	Match	(covered by setup rotation)
7	Must play if you legally can; no strategic pass	[W BR3][MTR]	Enforced (beacon/pass blocked while a chart exists)	Match	`Play — must play when able`
8	When stuck: draw exactly one; play it if possible; else marker; one draw/turn	[W r4][UBG][DP]	Enforced; draw + forced marker resolves atomically	Match	`Play — draw one, then play it or mark`
9	Your marker clears only when you play on your own train	[W][UBG]	Own-trail chart clears; NZ/opponent leaves it up	Match	`Play — marker clears only on your own trail`
10	You may play on a rival train only while its marker is down	[W][UBG][DP]	Enforced	Match	`Play — an opponent's trail is playable only while marked`
11	A played double must be covered before any other route is playable	[W][MTR][UBG][DP]	Only the cover play is legal while Red Alert is open	Match	`Doubles — an open double blocks all other routes`
12	The cover tile must be played on the double	[W] (casual [GC][DP][walnut] allow it anywhere)	Cover must be on the double	Choice (follows Wikipedia/tournament)	`Doubles — an open double blocks all other routes`
13	Responsibility to cover passes player-to-player; each failer marks their train	[W][UBG][MTR][DP]	Pass Red Alert deploys beacon, moves responsibility	Match	`rules-worked-examples.spec.ts` + `Red Alert pass`
14	A “dead” double (last tile of that pip) need not be covered	[W][MTR]	`isRedAlertDoubleDead` clears the alert	Match	`Doubles — a dead double need not be covered`
15	Only one double resolved per turn (no chaining)	[W][MTR] tournament (casual [GC][DP] allow chaining)	One double per turn	Choice (tournament)	`random-play-harness` (never chains)
16	You cannot go out on an unsatisfied double	[W][MTR] (casual [UBG][DP] allow it)	Empty hand does not win with an open double	Choice (tournament)	`Doubles — you cannot go out on an unsatisfied double`
17	Round ends when a player empties their hand	[W][UBG]	Round ends, winner recorded	Match	`Doubles — you cannot go out…` (negative) + fuzz completions
18	Round also ends when the boneyard is empty and no one can play (blocked)	[W][UBG]	Blocked sector; everyone scores	Match	`Scoring — blocked sector scores every captain`
19	Winner scores 0; others score the pip sum of their hand	[W][MoG]	Enforced	Match	`Scoring — winner scores 0, others sum their pips`
20	Blocked round: no exemption — every player scores their hand	[W][MTR]	Enforced	Match	`Scoring — blocked sector scores every captain`
21	Lowest cumulative total after all rounds wins	[W]	Points campaign winner = min cumulative	Match	round-13 completion + `self-play` winner logic
22	A tile counts both ends; a double counts both halves	[W][MoG]	`coordinatePipValue`	Match	scoring tests (9-9 = 18, etc.)

Documented divergences (Warp 12 house standard)

These are the points where Warp 12 intentionally departs from some online sources. All are within the range of common practice and are chosen to match tournament-style play; each is enforced by a test so the choice cannot silently regress.

7–8 player hand size = 10 by default, host-configurable to 11. This is the one setup value where major sources genuinely conflict — Masters of Games and most modern/commercial sets deal 10, while Galt 1994 and University Games deal
1. Warp 12 defaults to 10 (10-tile boneyard at 8 players vs only 2 at 11) and exposes a largeFleetHandSize house rule so hosts can opt into 11. Surfaced in the setup UI (local, pass-and-play, and online lobby) only for 7–8 captain fleets.
Cover must be played on the double (not “anywhere on the layout”). Follows Wikipedia/tournament; several casual sources allow the second tile anywhere.
One double resolved per turn (no chaining multiple doubles in a turn). Tournament-style; some casual rules allow chains.
No going out on an unsatisfied double. Tournament-style; UltraBoardGames and domino-play permit a final-double go-out.
Spacedock set aside before the deal + rotating starter. Wikipedia lists this as a standard method; the Galt 1994 alternative deals the engine double and lets its holder open. Warp 12 uses the set-aside method for every round so the round double is deterministic and never sits in a hand.
Tie handling. Wikipedia breaks ties (most 0-point rounds, then lowest non-zero round); Warp 12 declares a shared victory on an exact cumulative-total tie (RULES.md V, step 6). This is a presentation choice for a digital leaderboard, not a play-rule difference.

Warp 12–specific layers (out of scope for standard conformance)

RULES.md Sections VI–IX (Continuum, Salamander Penalty, Subspace Fracture, Drop to Impulse catch window, All Stop! ceremony, Manual shield control, TEI, Subspace messaging) are Warp 12 inventions or opt-in modules with no external standard to validate against. They are covered by their own unit tests and by the fuzz harness (which enables every module/house-rule combination and still holds tile conservation and turn-order invariants).