In message ,
Colin Rosenstiel writes
Most conveniently, what they do with some trains at the French-
Spanish border: slide the wheels along the axles to fit the other
gauge. Other solutions include mixed-gauge track, bogie changing,
and (of course) having the passengers change trains.

In the latter option it seems a bit pointless going to the expense of
building a tunnel.
According to my Jane's world railways the high speed line into Madrid is
standard gauge, not the normal Spanish broad gauge.
--
Clive.