diff --git a/lib/ship-queue-train-land.ts b/lib/ship-queue-train-land.ts
index d8d248529..c3d0e8e92 100644
--- a/lib/ship-queue-train-land.ts
+++ b/lib/ship-queue-train-land.ts
@@ -56,6 +56,7 @@ export async function trainLand(ctx: QueueContext, o: { train: string; pr: strin
   };
   const rerun = `${Q} train land --train ${t.nonce} --pr ${o.pr}`;
   const lines = [`TRAIN_LAND: ${t.nonce} slot ${i + 1}/${t.slots.length} #${o.pr} pin ${pin} -> S${s.index} v${s.version} tree ${s.tree}`];
+  if (git(root, ['rev-parse', `${s.commit}^{tree}`]).out !== s.tree || !git(root, ['merge-base', '--is-ancestor', s.commit, t.head]).ok) return outRefused({ code: 'QUEUE_PROOF_MISSING', message: `the record's S${s.index} is not a commit with tree ${s.tree.slice(0, 12)} on the gated head ${t.head.slice(0, 12)}` }, lines, `${Q} train status --train ${t.nonce}`, run);
   let main = mainNow();
   if ('error' in main) return outRefused({ code: 'QUEUE_PROOF_MISSING', message: main.error }, lines, rerun, run);
   if (main.tree === s.tree) return confirmMerged(env, t, o.pr, main.sha, s.index, [...lines, `main: ${main.sha} is S${s.index} exactly`]);
@@ -105,6 +106,7 @@ export async function trainLand(ctx: QueueContext, o: { train: string; pr: strin
   if (!m.ok) return outFailed({ code: 'QUEUE_NOT_READY', message: `gh pr merge refused: ${(m.err ?? m.out).trim().slice(0, 200)}` }, lines, rerun, run);
   main = mainNow();
   if ('error' in main) return outPending({ code: 'QUEUE_PROOF_MISSING', message: `merged; ${main.error}` }, lines, rerun, run);
+  if (main.sha === j.base) return outPending({ code: 'QUEUE_NOT_READY', message: `merged; main still reads ${j.base.slice(0, 12)}, so the merge is not visible yet` }, lines, rerun, run);
   if (main.tree !== s.tree) return stopped(env, t, 'QUEUE_TRAIN_TREE_MISMATCH', `after the merge main ${main.sha.slice(0, 12)} is not S${s.index} exactly`, main.sha, lines, true);
   return confirmMerged(env, t, o.pr, main.sha, s.index, [...lines, `MERGED: #${o.pr} as ${main.sha}; main's tree is S${s.index} exactly`]);
 }
diff --git a/lib/ship-queue-train-stamp.ts b/lib/ship-queue-train-stamp.ts
index 8c23c108b..4a7d7faee 100644
--- a/lib/ship-queue-train-stamp.ts
+++ b/lib/ship-queue-train-stamp.ts
@@ -83,8 +83,9 @@ export function stampChain(o: StampChainOptions): { stamped: StampedPrefix[]; li
       if (linked) return { code: 'QUEUE_TRAIN_STAMP_MISMATCH', message: `S${i + 1} (#${slot.pr}): ${linked} is a symlink in the merged tree; the stamp writes release files only inside the repository` };
       const r = restamp({ repoRoot: wt, base: o.base, baseRef: pred, next: true, policyFrom: 'base', allowRepoCommands: o.allowRepoCommands, dryRun: false, now: new Date(`${date}T12:00:00Z`), env: process.env });
       if (r.status !== 'stamped') return { code: r.code ?? 'QUEUE_TRAIN_STAMP_MISMATCH', message: `S${i + 1} (#${slot.pr}): restamp ${r.status}: ${r.message ?? ''}`, fix: r.fix };
-      git(wt, ['add', '-A']);
+      if (!git(wt, ['add', '-A']).ok) return { code: 'QUEUE_TRAIN_STAMP_MISMATCH', message: `cannot stage the stamp for S${i + 1}` };
       tree = git(wt, ['write-tree']).out;
+      if (!/^[0-9a-f]{40,64}$/.test(tree)) return { code: 'QUEUE_TRAIN_STAMP_MISMATCH', message: `cannot write the stamped tree for S${i + 1}` };
       version = r.newVersion!;
     } finally {
       git(o.repoRoot, ['worktree', 'remove', '--force', wt]);
diff --git a/test/ship-queue-train-land.test.ts b/test/ship-queue-train-land.test.ts
index eaf569e6d..b02e1936a 100644
--- a/test/ship-queue-train-land.test.ts
+++ b/test/ship-queue-train-land.test.ts
@@ -118,7 +118,7 @@ describe('stamped trains: build and land', () => {
     expect(line(drift.out, 'QUEUE_TRAIN_FROZEN:')).toMatch(/^QUEUE_TRAIN_FROZEN: train \S+ merged #1/);
   }, 300_000);
 
-  test('a stamp commit that cannot be written pushes nothing and leaves the PR branch in place', () => {
+  test('a record whose S_i tree is not on the gated chain pushes nothing and leaves the PR branch in place', () => {
     const { fx, h1, nonce } = stampedTrain();
     expect(queue(fx, ['train', 'verdict', '--train', nonce]).status).toBe(EXIT.ok);
     const file = path.join(fx.state, 'ship-queue', 'trains', nonce, 'train.json');
@@ -126,7 +126,7 @@ describe('stamped trains: build and land', () => {
     t.stamped[0].tree = 'f'.repeat(40);
     fs.writeFileSync(file, JSON.stringify(t));
     const r = land(fx, nonce, '1');
-    expect(r.status).not.toBe(EXIT.ok);
+    expect(r.err).toMatch(/is not a commit with tree ffffffffffff on the gated head.*\(QUEUE_PROOF_MISSING\)/);
     expect(r.out).not.toMatch(/^PUSHED:/m);
     expect(fx.repo.git('--git-dir', fx.repo.origin, 'rev-parse', 'refs/heads/pr-1')).toBe(h1);
   }, 180_000);