diff --git a/changelog.d/maintenance/6784-merge-train.md b/changelog.d/maintenance/6784-merge-train.md
new file mode 100644
index 0000000000..e98586183f
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+++ b/changelog.d/maintenance/6784-merge-train.md
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+- **Merge-train script** (`scripts/release/merge-train.sh`): batch-validates N queued PRs as ONE merged result on the runner box — merges every queued PR into a throwaway worktree cut from the release tip, runs the fast-gates parity suite once, and prints the `--admin` evidence block per PR (merge-gates §7). Replaces O(N²) per-PR CI re-runs in merge-storms. Regression guard: `tests/unit/merge-train-plan.test.ts`.
diff --git a/scripts/release/merge-train.sh b/scripts/release/merge-train.sh
new file mode 100755
index 0000000000..217a66212e
--- /dev/null
+++ b/scripts/release/merge-train.sh
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+#!/usr/bin/env bash
+# scripts/release/merge-train.sh — batch-validate N queued PRs as ONE merged result.
+#
+# Why: in a merge-storm, waiting for each PR's CI after each sibling merge costs
+# O(N²) CI runs. The train merges every queued PR into a throwaway worktree cut from
+# the release tip, runs the full fast-gates parity suite ONCE on the final result, and
+# prints the evidence block that authorizes `gh pr merge --squash --admin` for each
+# train member (merge-gates.md §7 — owner-approved policy extension of §4, 2026-07-09).
+#
+# Designed for the 32-core runner box (192.168.0.113) or any checkout with
+# node_modules. It only READS from origin — it never pushes, never merges PRs, never
+# touches other worktrees, and never uses `git stash` (Hard Rule #22a).
+#
+# Usage:
+#   scripts/release/merge-train.sh [--plan] <base-branch> <PR#> [<PR#>...]
+#     --plan   print the planned steps and exit 0 (no worktree, no network) — used by
+#              the unit test and for a quick sanity read.
+#
+# Exit codes: 0 = suite green (evidence printed); 1 = usage error; 2 = suite red;
+#             PRs whose merge conflicts are EJECTED (reported, train continues).
+set -euo pipefail
+
+PLAN=0
+if [ "${1:-}" = "--plan" ]; then
+  PLAN=1
+  shift
+fi
+
+if [ $# -lt 2 ]; then
+  echo "usage: $0 [--plan] <base-branch> <PR#> [<PR#>...]" >&2
+  exit 1
+fi
+
+BASE="$1"
+shift
+PRS=("$@")
+for N in "${PRS[@]}"; do
+  case "$N" in
+    ''|*[!0-9]*) echo "error: PR number '$N' is not numeric" >&2; exit 1 ;;
+  esac
+done
+
+ROOT="$(git rev-parse --show-toplevel 2>/dev/null || true)"
+SUITE=(
+  "npm run typecheck:core"
+  "node scripts/check/check-file-size.mjs"
+  "node scripts/check/check-complexity.mjs"
+  "node scripts/check/check-cognitive-complexity.mjs"
+  "node scripts/check/check-changelog-integrity.mjs"
+  "TEST_SHARD=1/2 npm run test:unit:ci:shard"
+  "TEST_SHARD=2/2 npm run test:unit:ci:shard"
+  "npm run test:vitest"
+)
+
+if [ "$PLAN" = "1" ]; then
+  echo "[merge-train] PLAN — base=origin/${BASE} prs=${PRS[*]}"
+  echo "[merge-train] 1. worktree add .claude/worktrees/merge-train-<ts> --detach origin/${BASE}"
+  for N in "${PRS[@]}"; do
+    echo "[merge-train] 2. fetch origin pull/${N}/head && merge (conflict → EJECT #${N}, continue)"
+  done
+  i=3
+  for c in "${SUITE[@]}"; do
+    echo "[merge-train] ${i}. ${c}"
+    i=$((i + 1))
+  done
+  echo "[merge-train] ${i}. green → print --admin evidence per PR; red → exit 2 (bisect + eject)"
+  echo "[merge-train] ${i}. teardown: git worktree remove --force (trap EXIT)"
+  exit 0
+fi
+
+if [ -z "$ROOT" ]; then
+  echo "error: not inside a git checkout" >&2
+  exit 1
+fi
+
+TS="$(date +%Y%m%d-%H%M%S)"
+WT="$ROOT/.claude/worktrees/merge-train-$TS"
+LOG="$WT-suite.log"
+
+cleanup() {
+  git -C "$ROOT" worktree remove --force "$WT" 2>/dev/null || true
+}
+trap cleanup EXIT
+
+echo "[merge-train] fetching origin/${BASE}…"
+git -C "$ROOT" fetch origin "$BASE" --quiet
+git -C "$ROOT" worktree add --detach "$WT" "origin/$BASE" --quiet
+# reuse the main checkout's node_modules (same convention as dev worktrees)
+[ -e "$WT/node_modules" ] || ln -s "$ROOT/node_modules" "$WT/node_modules"
+
+EJECTED=()
+BOARDED=()
+for N in "${PRS[@]}"; do
+  echo "[merge-train] boarding #${N}…"
+  if ! git -C "$WT" fetch origin "pull/${N}/head" --quiet; then
+    echo "[merge-train] ✗ #${N} EJECTED — could not fetch pull/${N}/head"
+    EJECTED+=("$N")
+    continue
+  fi
+  if git -C "$WT" merge FETCH_HEAD --no-edit --quiet >/dev/null 2>&1; then
+    BOARDED+=("$N")
+  else
+    git -C "$WT" merge --abort 2>/dev/null || true
+    echo "[merge-train] ✗ #${N} EJECTED — merge conflict vs the train (route it through the normal §5 path)"
+    EJECTED+=("$N")
+  fi
+done
+
+if [ ${#BOARDED[@]} -eq 0 ]; then
+  echo "[merge-train] no PR boarded — nothing to validate." >&2
+  exit 1
+fi
+
+TIP="$(git -C "$WT" rev-parse HEAD)"
+EJ_MSG=""
+[ ${#EJECTED[@]} -gt 0 ] && EJ_MSG=" — ejected: ${EJECTED[*]}"
+echo "[merge-train] train tip ${TIP} — boarded: ${BOARDED[*]}${EJ_MSG}"
+echo "[merge-train] running parity suite (log: ${LOG})…"
+
+for c in "${SUITE[@]}"; do
+  echo "[merge-train] ▶ ${c}"
+  if ! (cd "$WT" && eval "$c") >>"$LOG" 2>&1; then
+    echo "[merge-train] ✗ SUITE RED at: ${c}" >&2
+    echo "[merge-train] tail of ${LOG}:" >&2
+    tail -30 "$LOG" >&2
+    echo "[merge-train] bisect: re-run the failing gate on intermediate train commits, eject the offender, re-run." >&2
+    exit 2
+  fi
+done
+
+echo "[merge-train] ✅ SUITE GREEN on ${TIP}"
+echo "[merge-train] evidence line for each PR (paste before gh pr merge --squash --admin):"
+for N in "${BOARDED[@]}"; do
+  echo "  #${N}: Validated in local merge-train ${LOG} on $(hostname) @ ${TIP} (suite green)"
+done
+[ ${#EJECTED[@]} -gt 0 ] && echo "[merge-train] ejected (need the normal path): ${EJECTED[*]}"
+exit 0
diff --git a/tests/unit/merge-train-plan.test.ts b/tests/unit/merge-train-plan.test.ts
new file mode 100644
index 0000000000..770434d22d
--- /dev/null
+++ b/tests/unit/merge-train-plan.test.ts
@@ -0,0 +1,59 @@
+// Guards scripts/release/merge-train.sh (merge-gates.md §7 — batch validation of N
+// queued PRs as one merged result, replacing O(N²) per-PR CI re-runs). Only the
+// side-effect-free surface is testable in unit scope: --plan mode (no worktree, no
+// network) and argument validation.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+import { join, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const pExecFile = promisify(execFile);
+const SCRIPT = join(dirname(fileURLToPath(import.meta.url)), "../../scripts/release/merge-train.sh");
+
+async function run(args: string[]) {
+  try {
+    const { stdout, stderr } = await pExecFile("bash", [SCRIPT, ...args]);
+    return { code: 0, stdout, stderr };
+  } catch (err) {
+    const e = err as { code?: number; stdout?: string; stderr?: string };
+    return { code: e.code ?? -1, stdout: e.stdout ?? "", stderr: e.stderr ?? "" };
+  }
+}
+
+test("--plan prints the full step plan without touching anything and exits 0", async () => {
+  const { code, stdout } = await run(["--plan", "release/v9.9.9", "111", "222"]);
+  assert.equal(code, 0);
+  assert.match(stdout, /PLAN — base=origin\/release\/v9\.9\.9 prs=111 222/);
+  assert.match(stdout, /worktree add \.claude\/worktrees\/merge-train-/);
+  assert.match(stdout, /pull\/111\/head/);
+  assert.match(stdout, /pull\/222\/head/);
+  // the parity suite is fully enumerated in the plan
+  for (const gate of [
+    "typecheck:core",
+    "check-file-size.mjs",
+    "check-complexity.mjs",
+    "check-cognitive-complexity.mjs",
+    "check-changelog-integrity.mjs",
+    "TEST_SHARD=1/2",
+    "TEST_SHARD=2/2",
+    "test:vitest",
+  ]) {
+    assert.ok(stdout.includes(gate), `plan must include ${gate}`);
+  }
+  assert.match(stdout, /--admin evidence/);
+  assert.match(stdout, /teardown: git worktree remove/);
+});
+
+test("usage error without enough args", async () => {
+  const { code, stderr } = await run(["--plan", "release/v9.9.9"]);
+  assert.equal(code, 1);
+  assert.match(stderr, /usage:/);
+});
+
+test("rejects a non-numeric PR ref", async () => {
+  const { code, stderr } = await run(["--plan", "release/v9.9.9", "12a"]);
+  assert.equal(code, 1);
+  assert.match(stderr, /not numeric/);
+});