Hundreds of billions of dollars are being spent on railway construction worldwide as countries improve transportation options for passengers and freight. These long-distance connections often traverse mountain ranges, waterways or dense urban environments, requiring expensive bridge, viaduct and tunnel solutions.

Construction costs vary wildly – especially for high-speed rail – from an average cost per kilometre of $18.5 million (£15m) in Spain to a whopping $175 million (£139m) in the UK, according to a 2023 report from the Spanish government. What's more, budget and schedule overruns are typical for these complex and extensive engineering works.

Read on to discover the biggest railway megaprojects currently under construction (ranked in order of cost) and find out when they'll be completed.

All dollar amounts in US dollars.