diff --git a/specs/port/trios-railway/crates/trios-railway-experience/src/lib.t27 b/specs/port/trios-railway/crates/trios-railway-experience/src/lib.t27
new file mode 100644
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+// The append-only experience writer for .trinity/experience/.
+// Port of gHashTag/trios-railway:crates/trios-railway-experience/src/lib.rs
+// (at 8b229e9489ee, 140+ lines) — Roadmap Stage 6, goal #4548, target t27c
+// gen-rust.
+//
+// The original enforces L7 (the experience log) and L21 (context
+// immutability): the writer only ever opens the daily file in append mode
+// and never seeks or truncates — a caller that would truncate has no
+// method to call. An ExperienceLine is built from a RailwayHash only when
+// its phi_step is one of the eleven allowed PHI LOOP steps (mirroring
+// NOW.json); any other step is rejected. The file layout is fixed: the
+// experience directory under the root, the daily file named
+// trios_railway_<YYYYMMDD>.trinity, one serialized JSON line per append,
+// each with a trailing newline.
+//
+// The filesystem, the clock, the JSON serializer and the RailwayHash are
+// plumbing; the decisions — the eleven-step catalogue and the membership
+// gate, the line's built/invalid verdict with its carried fields and the
+// absent evidence, and the file layout rules with the append-only
+// contract — are ported here as pure functions and tested, including the
+// original's own rejects_unknown_phi_step test. The clock and the hash
+// triplet arrive as parameters (Utc::now and hash.triplet() are the
+// caller's plumbing), and the append_line transport keeps its name with
+// an `undefined;` body that no test calls (issue #9769, "Port the
+// decision, not the plumbing").
+
+module port::trios_railway_experience_lib;
+
+// ---------------------------------------------------------------------------
+// Original lines 18-31: `pub const PHI_STEPS` — the eleven allowed PHI
+// LOOP steps, mirroring NOW.json.
+// ---------------------------------------------------------------------------
+
+pub fn phi_step_count() -> usize {
+    return 11;
+}
+
+pub fn phi_step_in_order(index: usize) -> str {
+    if (index == 0) { return "CLAIM"; }
+    if (index == 1) { return "NAME"; }
+    if (index == 2) { return "SPEC"; }
+    if (index == 3) { return "SEAL"; }
+    if (index == 4) { return "GEN"; }
+    if (index == 5) { return "TEST"; }
+    if (index == 6) { return "VERDICT"; }
+    if (index == 7) { return "EXPERIENCE"; }
+    if (index == 8) { return "REPORT"; }
+    if (index == 9) { return "COMMIT"; }
+    if (index == 10) { return "PUSH"; }
+    return "";
+}
+
+// Original line 57: `if !PHI_STEPS.contains(&phi_step) { bail!(...) }` —
+// the membership gate.
+pub fn is_valid_phi_step(step: str) -> bool {
+    var i: usize = 0;
+    while (i < phi_step_count()) {
+        if (step == phi_step_in_order(i)) {
+            return true;
+        }
+        i += 1;
+    }
+    return false;
+}
+
+// ---------------------------------------------------------------------------
+// Original lines 33-72: `pub struct ExperienceLine` and
+// `pub fn from_hash(...) -> Result<Self>`. The line carries the audit
+// facts; the evidence is None (skip_serializing_if) — carried as the
+// absent flag. The clock (Utc::now) and the hash triplet arrive as
+// parameters; the built flag is the Ok/bail verdict.
+// ---------------------------------------------------------------------------
+
+pub struct ExperienceLine {
+    built: bool,
+    ts: str,
+    agent: str,
+    soul_name: str,
+    issue: str,
+    task: str,
+    status: str,
+    phi_step: str,
+    triplet: str,
+    evidence_present: bool,
+}
+
+pub fn ExperienceLine_from_hash(
+    agent: str,
+    soul_name: str,
+    issue: str,
+    task: str,
+    status: str,
+    phi_step: str,
+    triplet: str,
+    ts: str,
+) -> ExperienceLine {
+    if (!is_valid_phi_step(phi_step)) {
+        return ExperienceLine {
+            built: false,
+            ts: "",
+            agent: "",
+            soul_name: "",
+            issue: "",
+            task: "",
+            status: "",
+            phi_step: phi_step,
+            triplet: "",
+            evidence_present: false,
+        };
+    }
+    return ExperienceLine {
+        built: true,
+        ts: ts,
+        agent: agent,
+        soul_name: soul_name,
+        issue: issue,
+        task: task,
+        status: status,
+        phi_step: phi_step,
+        triplet: triplet,
+        evidence_present: false,
+    };
+}
+
+// ---------------------------------------------------------------------------
+// Original lines 74-98: `pub async fn append_line(root, line) ->
+// Result<PathBuf>`. The append-only writer: creates the experience
+// directory, opens the daily file with create(true).append(true) — never
+// truncating, per L21 — writes the serialized line with its trailing
+// newline, and returns the path. Pure transport; the layout rules it
+// fixes are the decisions, ported below.
+// ---------------------------------------------------------------------------
+
+pub fn append_line(root: str, day: str) -> ?str {
+    return undefined; // plumbing: append-only write to the daily experience file (lines 74-98)
+}
+
+pub fn experience_dir_name() -> str {
+    return "experience";
+}
+
+pub fn daily_file_prefix() -> str {
+    return "trios_railway_";
+}
+
+pub fn daily_file_suffix() -> str {
+    return ".trinity";
+}
+
+pub fn day_format() -> str {
+    return "%Y%m%d";
+}
+
+// Original lines 88-90: `OpenOptions::new().create(true).append(true)` —
+// append-only, never truncate (L21: no truncating method exists).
+pub fn opens_append_only() -> bool {
+    return true;
+}
+
+// Original line 86: `serialized.push('\n')` — every appended line ends
+// with a newline, so the file is line-per-record.
+pub fn line_ends_with_newline() -> bool {
+    return true;
+}
+
+// ---------------------------------------------------------------------------
+// Tests: every decision above asserted against values read from the
+// original's own tests (lines 105-140). No test calls the append_line
+// transport.
+// ---------------------------------------------------------------------------
+
+test valid_step_builds_the_line {
+    const line = ExperienceLine_from_hash("GENERAL", "DustyDeployer", "#1", "bootstrap", "OK", "EXPERIENCE", "triplet-x", "2026-10-17T02:39:00Z");
+    assert(line.built);
+    assert(line.agent == "GENERAL");
+    assert(line.soul_name == "DustyDeployer");
+    assert(line.issue == "#1");
+    assert(line.task == "bootstrap");
+    assert(line.status == "OK");
+    assert(line.phi_step == "EXPERIENCE");
+    assert(line.triplet == "triplet-x");
+    assert(!line.evidence_present);
+}
+
+test rejects_unknown_phi_step {
+    const line = ExperienceLine_from_hash("GENERAL", "DustyDeployer", "#1", "bootstrap", "OK", "RUN", "triplet-x", "2026-10-17T02:39:00Z");
+    assert(!line.built);
+}
+
+test phi_steps_catalog {
+    assert(phi_step_count() == 11);
+    assert(phi_step_in_order(0) == "CLAIM");
+    assert(phi_step_in_order(6) == "VERDICT");
+    assert(phi_step_in_order(7) == "EXPERIENCE");
+    assert(phi_step_in_order(10) == "PUSH");
+    assert(phi_step_in_order(11) == "");
+}
+
+test membership_gate {
+    assert(is_valid_phi_step("CLAIM"));
+    assert(is_valid_phi_step("EXPERIENCE"));
+    assert(is_valid_phi_step("COMMIT"));
+    assert(is_valid_phi_step("PUSH"));
+    assert(!is_valid_phi_step("RUN"));
+    assert(!is_valid_phi_step(""));
+}
+
+test daily_file_layout_and_append_contract {
+    assert(experience_dir_name() == "experience");
+    assert(daily_file_prefix() == "trios_railway_");
+    assert(daily_file_suffix() == ".trinity");
+    assert(day_format() == "%Y%m%d");
+    assert(opens_append_only());
+    assert(line_ends_with_newline());
+}