diff --git a/tests/testRail.cpp b/tests/testRail.cpp
new file mode 100644
index 0000000..a08f274
--- /dev/null
+++ b/tests/testRail.cpp
@@ -0,0 +1,205 @@
+#include "testHarness.h"
+#include "vf/Rail.h"
+
+using namespace vf;
+
+namespace {
+
+void placeRail(World& world, int x, int y, int z, Block kind = Block::Rail) {
+    world.setBlock(x, y, z, kind);
+}
+
+} // namespace
+
+VF_TEST(straightRailInferredFromOppositeNeighbours) {
+    World world;
+    placeRail(world, 0, 0, 0);
+    placeRail(world, 0, 0, 1);
+    placeRail(world, 0, 0, -1);
+    CHECK(inferRailShape(world, {0, 0, 0}) == RailShape::NorthSouth);
+
+    World ew;
+    placeRail(ew, 0, 0, 0);
+    placeRail(ew, 1, 0, 0);
+    placeRail(ew, -1, 0, 0);
+    CHECK(inferRailShape(ew, {0, 0, 0}) == RailShape::EastWest);
+}
+
+VF_TEST(loneRailDefaultsToNorthSouth) {
+    World world;
+    placeRail(world, 0, 0, 0);
+    CHECK(inferRailShape(world, {0, 0, 0}) == RailShape::NorthSouth);
+}
+
+VF_TEST(plainRailCurvesBetweenPerpendicularNeighbours) {
+    World world;
+    placeRail(world, 0, 0, 0);
+    placeRail(world, 0, 0, 1);   // south
+    placeRail(world, 1, 0, 0);   // east
+    CHECK(inferRailShape(world, {0, 0, 0}) == RailShape::CurveSouthEast);
+}
+
+VF_TEST(poweredRailNeverCurves) {
+    World world;
+    placeRail(world, 0, 0, 0, Block::PoweredRail);
+    placeRail(world, 0, 0, 1);   // south
+    placeRail(world, 1, 0, 0);   // east
+    RailShape shape = inferRailShape(world, {0, 0, 0});
+    CHECK(shape == RailShape::NorthSouth || shape == RailShape::EastWest);
+}
+
+VF_TEST(ascendingShapeInferredFromRaisedNeighbour) {
+    World world;
+    placeRail(world, 0, 0, 0);
+    placeRail(world, -1, 0, 0);   // west, flat
+    placeRail(world, 1, 1, 0);    // east, raised
+    CHECK(inferRailShape(world, {0, 0, 0}) == RailShape::AscendEast);
+}
+
+VF_TEST(advanceFollowsStraightTrack) {
+    World world;
+    for (int x = 0; x <= 3; ++x) placeRail(world, x, 0, 0);
+    RailStep step = advanceOnRail(world, {1, 0, 0}, RailDir::East);
+    CHECK(step.valid);
+    CHECK(step.nextPos == Vec3i(2, 0, 0));
+    CHECK(step.exitDir == RailDir::East);
+}
+
+VF_TEST(advanceTurnsThroughCurve) {
+    World world;
+    placeRail(world, 0, 0, 0);   // curve SE
+    placeRail(world, 0, 0, 1);   // south neighbour
+    placeRail(world, 1, 0, 0);   // east neighbour
+    RailStep step = advanceOnRail(world, {0, 0, 0}, RailDir::North);
+    CHECK(step.valid);
+    CHECK(step.exitDir == RailDir::East);
+    CHECK(step.nextPos == Vec3i(1, 0, 0));
+}
+
+VF_TEST(advanceClimbsAndDescendsSlopes) {
+    World world;
+    placeRail(world, 0, 0, 0);
+    placeRail(world, -1, 0, 0);
+    placeRail(world, 1, 1, 0);
+
+    RailStep up = advanceOnRail(world, {0, 0, 0}, RailDir::East);
+    CHECK(up.valid);
+    CHECK(up.nextPos == Vec3i(1, 1, 0));
+
+    RailStep down = advanceOnRail(world, {1, 1, 0}, RailDir::West);
+    CHECK(down.valid);
+    CHECK(down.nextPos == Vec3i(0, 0, 0));
+}
+
+VF_TEST(advanceDerailsAtTrackEnd) {
+    World world;
+    placeRail(world, 0, 0, 0);
+    placeRail(world, 1, 0, 0);
+    RailStep step = advanceOnRail(world, {1, 0, 0}, RailDir::East);
+    CHECK(!step.valid);
+}
+
+VF_TEST(switchedJunctionTakesTheTurn) {
+    World world;
+    placeRail(world, 0, 0, 0);
+    placeRail(world, 0, 0, -1);   // north
+    placeRail(world, 0, 0, 1);    // south
+    placeRail(world, 1, 0, 0);    // east branch
+    CHECK(inferRailShape(world, {0, 0, 0}, false) == RailShape::NorthSouth);
+    CHECK(inferRailShape(world, {0, 0, 0}, true) == RailShape::CurveSouthEast);
+}
+
+VF_TEST(poweredRailPropagatesWithReachFalloff) {
+    World world;
+    std::vector<Vec3i> rails;
+    for (int x = 0; x <= 12; ++x) {
+        world.setBlock(x, 0, 0, Block::PoweredRail);
+        rails.push_back({x, 0, 0});
+    }
+
+    auto active = activePoweredRails(world, rails, [](const Vec3i& p) {
+        return p == Vec3i(0, 0, 0);
+    });
+
+    CHECK(active.count({0, 0, 0}) == 1);
+    CHECK(active.count({8, 0, 0}) == 1);
+    CHECK(active.count({9, 0, 0}) == 0);
+    CHECK(active.count({12, 0, 0}) == 0);
+}
+
+VF_TEST(poweredRailPropagatesBothDirectionsFromMiddleSource) {
+    World world;
+    std::vector<Vec3i> rails;
+    for (int x = -10; x <= 10; ++x) {
+        world.setBlock(x, 0, 0, Block::PoweredRail);
+        rails.push_back({x, 0, 0});
+    }
+
+    auto active = activePoweredRails(world, rails, [](const Vec3i& p) {
+        return p == Vec3i(0, 0, 0);
+    });
+
+    CHECK(active.count({8, 0, 0}) == 1);
+    CHECK(active.count({-8, 0, 0}) == 1);
+    CHECK(active.count({9, 0, 0}) == 0);
+    CHECK(active.count({-9, 0, 0}) == 0);
+}
+
+VF_TEST(unpoweredRailNetworkStaysInactive) {
+    World world;
+    std::vector<Vec3i> rails;
+    for (int x = 0; x <= 5; ++x) {
+        world.setBlock(x, 0, 0, Block::PoweredRail);
+        rails.push_back({x, 0, 0});
+    }
+    auto active = activePoweredRails(world, rails, [](const Vec3i&) { return false; });
+    CHECK(active.empty());
+}
+
+VF_TEST(railGraphMeasuresDistanceAlongLine) {
+    World world;
+    std::vector<Vec3i> cells;
+    for (int x = 0; x <= 5; ++x) {
+        placeRail(world, x, 0, 0);
+        cells.push_back({x, 0, 0});
+    }
+    RailGraph graph;
+    graph.build(world, cells);
+
+    CHECK_EQ(static_cast<int>(graph.size()), 6);
+    CHECK(graph.contains({3, 0, 0}));
+    CHECK(!graph.contains({100, 0, 0}));
+    CHECK_EQ(graph.distance({0, 0, 0}, {5, 0, 0}), 5);
+    CHECK_EQ(graph.distance({0, 0, 0}, {100, 0, 0}), -1);
+    CHECK_EQ(static_cast<int>(graph.neighbours({0, 0, 0}).size()), 1);
+    CHECK_EQ(static_cast<int>(graph.neighbours({3, 0, 0}).size()), 2);
+}
+
+VF_TEST(railGraphTracesAClosedLoop) {
+    World world;
+    std::vector<Vec3i> cells = {{0, 0, 0}, {1, 0, 0}, {1, 0, 1}, {0, 0, 1}};
+    for (const Vec3i& c : cells) placeRail(world, c.x, c.y, c.z);
+
+    RailGraph graph;
+    graph.build(world, cells);
+
+    std::vector<Vec3i> path = graph.trace(world, {0, 0, 0}, RailDir::North, 20);
+    CHECK_EQ(static_cast<int>(path.size()), 4);
+    CHECK_EQ(graph.distance({0, 0, 0}, {1, 0, 1}), 2);
+    CHECK_EQ(static_cast<int>(graph.neighbours({0, 0, 0}).size()), 2);
+}
+
+VF_TEST(railGraphTraceStopsAtDeadEnd) {
+    World world;
+    std::vector<Vec3i> cells;
+    for (int x = 0; x <= 3; ++x) {
+        placeRail(world, x, 0, 0);
+        cells.push_back({x, 0, 0});
+    }
+    RailGraph graph;
+    graph.build(world, cells);
+
+    std::vector<Vec3i> path = graph.trace(world, {0, 0, 0}, RailDir::East, 20);
+    CHECK_EQ(static_cast<int>(path.size()), 4);
+    CHECK(path.back() == Vec3i(3, 0, 0));
+}