- Saddle tank: A tank locomotive with the water tank mounted on top of the boiler like a saddle.
- Safety Appliance Act (US): A law mandating air brakes, grab bars, and automatic couplers
- Safeworking: The system of rules and equipment designed to ensure the safe operation of trains.
- Sand: granular material poured on the rail in front of the drive wheels to improve traction. (Sandite is a more specialised form for a similar purpose.)
- Sandbox: A container on locomotives and self-propelled multiple units, or trams, that run on tramways and adhesion railways. The container holds sand which can be dropped on to the rail to improve rail adhesion under wet, steep or slippery rail conditions. The sandbox and operating mechanism are collectively known as 'sanding gear'.
- Sandite: consists of a mixture of sand, aluminium and a unique type of adhesive, used instead of plain sand for extreme slippery rail conditions.
- Santa Fe type: A steam locomotive with a 2-10-2 wheel arrangement, named for the Atchison, Topeka and Santa Fe Railway, the first railroad to use such a configuration.
- Saturated locomotive: A steam locomotive not equipped with a superheater (see); the steam thus remains at the same temperature as the water in the boiler.
- Scale: Solid debris distilled from boiling water in a steam locomotive. To prevent corrosion damage from scale build-up, the locomotive must undergo a boiler wash once each operating month.
- Schnabel car: A specialized type of freight car for extra heavy and over sized loads; the car is loaded in such a way that the load forms part of the car superstructure.
- Searchlight: A signal with a single light source usually capable of displaying three different colors. An internal mechanism governs the color displayed.
- Section: The division of the track for security (occupation).
- Semaphore: A type of signal that has a moving arm; any signalling using semaphores.
- Shay: A type of geared steam locomotive built to the patents of Ephraim Shay.
- Shoofly (US): A temporary stretch of track that takes trains around construction or an accident scene.
- Shunt (UK) (AUS): To move trains or vehicles from one track to another.
- Shunt (US): To bond the rails/power feeds between sections on trolley/light rail systems, so as to temporarily bridge past dead areas.
- Shunter (UK): switcher (US) or shifter (PRR only): A small locomotive used for assembling trains and moving railroad cars around. Also, a person involved in such work.
- Shuttle service: A train, usually a passenger service, that runs back and forth over a relatively short distance, such as between a junction station and a branch-line terminus.
- Side tank: A tank locomotive with water tanks mounted each side of the boiler.
- Siding: A section of track off the main line. Sidings are often used for storing rolling stock or freight. A siding is also used as a form of rail access for warehouses and other businesses, where the siding will often meet up with loading docks at rail car height in the building. In the U.S. the term is also used to cover the British term: loop. Also, a passing track in the U.S.
- Signal: A device that indicates to the driver of a train information about the line ahead.
- Signal aspect: The information conveyed to a railroad vehicle operator by a block signal. Signals may use colored lights, position-significant lights or mechanical semaphores to generate various aspects.
- Signal box: A building or room which houses signal levers (usually in a frame), a control panel or a VDU-based control system.
- Signal Passed At Danger or SPAD (UK): where a train disobeys a stop signal.
- Signalman: A person in charge of the signalling at a station or junction, often in a Signal-box.
- Six-foot: The narrow corridor between a pair of closely spaced tracks, nominally six feet wide. See also four-foot and ten-foot.
- Six-pack (slang, US): A six-axle diesel locomotive.
- Slack (UK): A temporary speed restriction to protect, for example, sections of track in poor condition and awaiting repair. Also applies to the timing tolerance included in timetable schedules to allow for such restrictions.
- Slack action (UK/US): Looseness in a train caused by mating clearances in couplers.
- Sleeper (UK), tie (US): Bars placed at 90° to the rail tracks to support the rails. Generally of wood, concrete or steel, with hardware to affix the rails, usually spikes, nails or bolts. Note in the UK baseplates and clips are used to affix the rail to the sleeper. Spikes are widely used in North America.
- Slip coach (UK): A passenger coach that is disconnected from a train without the train havin to stop. While the train continued on its route, the slip coach would be guided and stopped by a guard on board using the coach's own brake mechanism. This practice was almost entirely limited to the United Kingdom and was discontinued in the 1960's.
- Slippery rail: The condition of fallen leaves or other debris lying on and clinging to a railroad track that could cause train wheel slippage, resulting in premature wheel wear and train delays.
- Slow order: A local speed restriction below the track's normal speed limit often designated by yellow and green flags. Slow orders can be imposed on a temporary basis to protect, for example, maintenance of way employees while sections of track are under repair. Widely used in areas where track is substandard and in need of repair.
- Slug: A locomotive that contains traction motors yet lacks the diesel engine to create its own power, which is instead supplied by a connected 'mother' locomotive.
- Smokebox: An enclosed (normally cylindrical) space attached to the end of the boiler opposite the firebox on a steam locomotive (normally the front). Supports the stack/chimney; steam pipes to and from the cylinders pass through here; contains the blastpipe/exhaust nozzle where the exhaust steam is used to provide draft for the fire. In superheated locomotives, also contains the superheater header and (optionally) a front-end throttle. A smokebox door allows access for cleaning.
- Smokestack (abbr. stack) (US): chimney (UK).
- Snow plough (Snowplow) or rail snow plough is a rail service vehicle used for snow removal from train tracks.
- SPAD (UK): Signal Passed At Danger, where a train disobeys a stop signal. Sometimes referred to as a "blown red" in U.S. railroad slang.
- Span bolster: the beam between two bogies
- Speeder (US): A small vehicle used to let track inspectors and work crews move quickly to and from work sites. (Obsolete) Speeders have mostly been replaced by trucks and SUVs with retractable flanged wheels.
- Spike: A bolt, pin or nail used to hold rails, or plates connected to the rails (known as tie plates), to sleepers (ties). Commonly called a "Dog" or "Dogspike" in Australia.
- Spiral easement See Track transition curve. Also known as tangent lead-in.
- Spreader a maintenance of way equipment designed to spread or shape ballast profiles, remove snow, clean and dig ditches as well as trim embankments
- SPT (UK): Signal-post telephone - A direct no-dial telephone link to the relevant Signal-box, positioned on or near a signal.
- Spur (US): A stretch of rail that branches off the main line. Different from a siding or stub, spurs can be miles in length, and usually have only one destination at the end.
- SPURT (India): An acronym for Self Propelled Ultrasonic Rail Testing, a self-propelled rail defect detector car.
- Staff and ticket: A method of safeworking involving a token.
- Standard gauge: Railroad track where the rails are spaced 1,435 mm (4 ft 8 1⁄2 in) apart. This is by far the most common gauge of railway worldwide.
- Station-master: The person in charge of a station.
- Station pilot (UK): Shunting engine based at a major passenger station and used for passenger train shunting duties.
- Steam generator: A device generally used in passenger trains to create steam for heating. The steam generator is usually in the locomotive but may also be located in other cars.
- Steam reverser: A reversing gear worked by a steam piston controlled from the cab.
- Steeplecab (US): An electric locomotive with a central cab and sloping "noses" on each end.
- Steward: A person in a dining car with a role similar to that of a Maitre d’Hotel.
- Stretch braking: Pulling against train brakes at the lower end of a locomotive's power output, e.g. notches 1 through 4 of a conventional throttle, thus keeping coupler slack stretched and permitting smoother train handling. This is considered most effective on undulating track profiles or when dynamic braking is not available.
- Stub (North America) A relatively short section of track that ends at a bumper or wheelstop, most often found in a terminal. Not to be confused with a spur, which may be miles (kilometers) in length.
- Subway (UK): A tunnel passing underneath the railway tracks to allow passengers to cross from one platform to another.
- Subway (US): A railroad that runs underground, generally in a large city. Subways are also considered "heavy rail" because they operate on their own dedicated track. Not to be confused with the interurban definition of subway, which is normally a light-rail passenger service running mostly underground.
- Supercharger A mechanical device that boosts the pressure of engine intake air to above atmospheric level, causing an increase in power. Not to be confused with the blower used to scavenge the cylinders of a naturally aspirated two-stroke Diesel engine.
- Superelevation (UK): Synonymous with cant: the banking of railroad track on curves. Specifically, the practice on high speed lines (where the cant needs to be higher) of gently introducing the elevation of the outer rail before the bend starts, in order to avoid sudden lurches.
- Superheater: A device in a steam locomotive that raises the temperature of saturated steam substantially beyond the boiling point of water, increasing power and efficiency.
- Switch (US): points (UK). Also "turnout".
- Switcher (US), shunter (UK): A small locomotive used for assembling trains and moving railroad cars around.
- Switchman: A railroad worker responsible for assembling trains and switching railroad cars in a yard; now often used together with brakeman as a single job description ("brakeman/switchman").
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