> ...arriving in Milan on Feb 6 and plans to take the train that morning to Venice.
> She is concerned that she might need a reservation
It depends by which train she´s gonna take. If it is
a Eurostar the reservation is mandatory, and she can
buy ticket+seat until 20 min. before the train
departs. If on the other hand that is an Intercity
train, the reservation is not obligatory and she can
just get on and find a free seat. Note that a free
seat corresponds to an empty space on the board out of
the compartment, while if there´s some piece of paper
the seat is reserved between the cities written on it.
By rule, 20% of the total seats are not reserved, so
she might be able to find a free one unless the train
is very crowded (but I wouldn´t be worried about it).
Flavio in Copenhagen