I searched extensively for this term "túnel de ventana," which is not a very common one. However, I found that it is often associated with a "túnel de conducción." I found a paper that seems to give a description of the "túnel de ventana" as an access tunnel that connects to the "túnel de conducción" (transport tunnel), by referring to a "ventana de acceso" (literally "window of access" but better, "access opening") that is reached through a"túnel de ventana" which would therefore seem to be an "access tunnel." The paper is found here (reference on page 143): http://repositorio.uncp.edu.pe/bitstream/handle/UNCP/1366/TE...

The key text in Spanish: "Para la construcción se diseñó una ventana de acceso de 50.00 m. de longitud, a la que se accede a través de un ramal desde el Túnel de Ventana 2, Prog. 0+320 cruzando por el túnel de conducción, donde inicia el trazo de la chimenea de equilibrio superior."