Possible Clues:

• "Three Blind Mice" line, or what can be said of 20-, 29-, and 46-Across
• Consult rail timetable and watch detailed departures?
• Look how fast the trio go
• Look to their fasting, perhaps; the mice can't look to it themselves
• even though they're 13 across, look how fast they are
• Just look how fast 16 down is with 32 down!