I would say the Channel Tunnel, the Lotschberg (sp) tunnel in Switzerland (which is the longest land tunnel in the world) and the A4 class of steam loco, still the fastest in the world.

I'm pleased to see all these Swiss suggestions:D

Thanks to Albula and Macmatt for their proposals and for the motivations.
Of course, as far as Switzerland is concerned, we could add the Rhaetian Alps network, which has also applied to UNESCO as Heritage Site.
I would be careful to add any modern engineering wonder such as tunnels and high-speed lines.
Now the new Loetschberg base tunnel is an engineering wonder more than a historical railway wonder.
The Channel Tunnel and the Great Belt link and the Oresund link between Denmark and Sweden have broken the isolation of the respective Countries, and created an enlarged European Raiway Network.
Bye from Dario

Here is my list of seven favourite railway wonders.
1. Transiberian line Moscow to Vladivostock
2. Tibet link from China
3. Trans-Australia lines Melbourne/Sydney - Hobart/Perth
4. Chicago- S.Francisco lines (UP and SF, now Amtrak)
5. Lima-Oroya in Peru
6. Capetown-Johannesburg/Pretoria - S.Africa
7. Glacier Express Andermatt-St.Moritz - Switzerland
Ciao from Dario

Thanks to the 378 viewers so far which make this the most popular thread.
Ciao from Dario

Raurimu Spiral, New Zealand. Still very much in use.
Another way of gaining height, similar feat as the ZigZag Railway achieved.
Great piece of engineering skill when the New Zealand main trunk line was being first built .
The Main Trunk was completed in 1908.
Actually, I came across this Railway Forum list by chance, as I was searching to see if there was a book or DVD about great railway engineering
feats. Looks like either one might be of interest to many people.
AMC

Welcome to the Forum AMC. Look forward to more posts from you.

Best wishes,

John H-T.

Any heritage railway built or rebuilt by volunteers.
Richard

can't be bothered to think of 7, but I'm surprised these haven't been mentioned

West Highland line
MOB ascent out of Montreux
northern Spain narrow gauge

Yes, the Raurimu Spiral is certainly a feat of engineering. :D It's electrified these days.