1. In some countries (f.e. Spain), there has been a strict distinction between bored tunnels and cut-and-cover tunnels, the latter being called 'false tunnels'. However, nowadays tunnels often have both kinds of sections.

2. That's easy, but what do you count into the length of the tunnel at the St. Pancras end of the Channel Tunnel Rail Link? There, the bored tunnel is continued by closed tubes -- which also function as bridges above the tracks ino Kings Cross...

3. Subways are an issue, too: should the sections between stations count as a single tunnel, or an entire line - or the entire network, if there are lots of bifurcations and sections co-usded by multiple lines? Or just deny subway tunnels the title of 'tunnel'.

4. Even if you ditch subways, what about the in-construction Wienerwald and Lainzer tunnels into Vienna? The first ends not at the surface but a junction at the end of the second, so it's like one single tunnel with exits in the middle -- well, except for the other end of the second, which branches out in three directions...

5. As an opposite complication, consider the Inntaltunnel and the future Brenner Base Tunnel near Innsbruck. These are at right angles, but there will be an underground connection. The freight trains that shall use it will traverse the longest tunnel section in the world, but is it one tunnel, or two, or three?...

Basically. It can get more complicated -- some stops do have switches, some stations no longer have them... The distinction matters for signalling and dispatching, because in some countries, above all Germany, stations have different traffic rules than open lines (e.g. how a train can be stopped or sent on its way, how and when shunting can be done...).

_{*Lunatic*, n.
One whose delusions are out of fashion.}