Because there was no one standard back in the 19th century, not everything is standardised. You can have different loading gauges (roughly, what size vehicle wi

Because there was no one standard back in the 19th century, not everything is standardised. You can have different loading gauges (roughly, what size vehicle will fit through the infrastructure) and the track gauge (the width between the rails). Whilst most of continental Europe is fairly standardised nowadays, you have various exceptions, like Spain and Portugal using a metre wide track gauge, or Great Britain[1] having a very small loading gauge. Beyond that, the order size of the various railways is often relatively low (hundreds to thousands or carriages, or dozens of full sets) and railway operators often want them customised for specific services types (an intercity operator will want first class coaches and provision for food service, a commuter operator will want to cram as many people in as possible etc.) so everything going to be bespoke at some level. [1] The island of Ireland, both Northern Ireland and the Republic of Ireland, uses a different set of standards to Great Britain or mainland Europe.