Railway (Railroad in the USA):
A system with a fixed steel rail guideway (called "track") using flanged steel wheels.
|US freight train approaching the Cajon Pass. Photo by atsfherb.|
A railway between two separate centres of population or industry (e.g. London and Brighton or New York City and Albany). Can operate freight and/or passenger trains. Some routes are purpose-built for freight.
|Swiss SZU Commuter train passes Giesshuebel. Photo by John Weismann|
A route or service operating passenger trains between a city centre and residential suburbs over main line tracks. Many of the larger suburban systems use electric traction.
|Train at Park Kultury on the Moscow Metro. Photo studentsoftheworld|
Urban or Metro line (Subway in the US):
A high capacity passenger railway route, normally segregated from main line railways, operating between and through a city centre and suburbs around it. Central sections of such routes are often under ground or elevated. A metro can share tracks with main line railways if capacity allows it. Usually uses electric traction.
|A train on the "Metro Transit" light rail line in Minneapolis. Photo by Jeff Terry|
Light Rail System (LRT - Light Rail Transit):
A medium capacity passenger railway using streets for some or all of its routes. Some ambitious cities refer to such systems as "metros" but they are rarely underground (e.g. Brussels offering an exception to the rule).
Some anomalies exist in these definitions, like the rubber-tyred metro systems seen in cities in Paris, Mexico City and Montreal, which have a steel rail back-up system, and "People Movers" like those short low capacity systems seen in airports.
You can get a whole range of railway definitions from the railway-technical Glossary pages .