The Eurostar from London to Amsterdam has just launched.
Is this route built on a brand new railway track / infrastructure? Or is it simply using an exising network with updated links?
No. The reason it took so long for this route to appear was because the original Eurostar trains (e300/Class 373) don't support the new European standard in-cab signalling and automatic train protection system used in the Netherlands; nor do most units support the 1.5kV DC overhead electrification used on its conventional railways. However, now they have replaced many of the older trains with the new e320/Class 374 trains, which support both of these (along with the systems used in Germany, also unsupported by the original trains).
The trains use High Speed 1 from St. Pancras, before proceeding through the Channel Tunnel onto LGV Nord, then diverging onto HSL 1 towards Brussels, joining the conventional rail network just south of there until Antwerp, where it joins HSL 4 to the Dutch border, where it continues as HSL-Zuid to Rotterdam and Amsterdam (though it must use the Dutch conventional railways around the immediate vicinity of these two stations). The latest of these lines opened to passengers in 2009.