Necessity on a cross-country train trip ...or a hint to each of the follow-up clues attaches to 1717AWhere to fix cavities?[---------------]-, 3737AMonkey's uncle?[-------]-, and 62-Across62ATitle of UN agency heads[---------------]
22D
Fish in a Japanese garden
24D
Ostrich's Australian cousin
25D
Door buster
26D
Econ. yardstick
28D
Shocking fish
29D
New York scenery restaurant (according to "Wizards of Waverly Place")