I have heard that it can be difficult to book transportation (air or
train) from central Thailand to southern Thailand durnig the highest
tourist season of the year (December into Jan.). Has anyone had good
or bad luck traveling north to south during this time of the year? If
not by train or air then by bus?

We are heading to Thailand for 4 weeks of travel in December and
definitely wish to get to the southeastern islands about one week
before New Years.