Some call it the gravy train, the gift that keeps on giving. To others, it’s a load of crap, and they cuss it. Every day it piles higher, this avalanche of music that arrives in tastemakers’ offices across the country, requiring both thoughtful efficiency and a cold-hearted detachment to conquer. The gravy train, it just keeps rolling along, pulling a bottomless trough of free music delivered to journalists, radio programmers, music supervisors and entertainment editors. Filled with new CDs — advances of forthcoming releases, full-art copies of fresh music, box sets from major labels, CD-Rs from budding bands looking for a break — each unrecyclable mailer is its own little plea.