Technically the cost of the actual rail infrastructure is less. It requires much less width for the corridor and both road and rail need roughly equal strengthening for the foundations. Overhead lines and signalling are not a significant incremental cost so long as you have an existing network.

However, you generally only build rail where there is sufficient latent demand. This means the land you require is of higher value and land is one of the most significant costs assuming you would need equal infrastructure requirements (e.g. bridges, structures, tunnels, etc.) regardless of mode.

Therefore, on a per kilometre basis, rail is often more expensive. The key difference is throughput. Rail is highly efficient for both freight and passenger movements, over sufficient distances, because it is a fixed corridor with right of way or full segregation. A dual track railway can carry far more tonnes of freight or many multiples more people than a dual lane road as it doesn’t suffer from congestion.