Hidden document in seatpost!

Gino Bartali successfully used this trick during WWII to bring documents for partisans:

[snip]

Bartali used his fame to carry messages and documents

[snip]

His sons found from his diaries that Bartali had used his fame to help.

So did he actually use his frame, or just his fame? It doesn't specify it further in this source.

Bartali continued with the Assisi Underground. In 1943, he led Jewish refugees towards the Swiss Alps himself. He cycled pulling a wagon with a secret compartment, telling patrols it was just part of his training.

This is just mind-boggling. How could he possibly have got away with that? Was there a person/were there several persons hidden in the wagon?