In "The Whitsun Weddings," what does "larking" mean?"I took for porters larking with the mails ..."

"Larking" is a verb used in British English that refers to the
idea of joking around or messing around with something. Thus to "lark around" could be understood
as "goofing off" or "joking around." In the quote you have identified, the speaker is referring
to the sound that he hears and what he sees and the way that he interpreted it as being the
porters on the train messing around with the mail that they were either picking up or dropping
off at the station where they were. It is only later on that he understands that it is the sound
of a number of weddings as the train passes wedding parties that have met to see the new bride
and groom off, which leads on to his meditation and thoughts regarding the importance of
marriage, change and tradition.