Delray or Romulus seem to be the busiest spots if number of trains is what you're looking for.
PatC created a monster, 'cause nobody wants to see Don Simon no more they want AARR I'm chopped liver, well if you want AARR this is what I'll give ya, bad humor mixed with irrelevant info that'll make you roll your eyes quicker than a ~Z~ banhammer...
Romulus - Good mixture of CSX, NS, and CP action. Great in the morning (especially weekends), but dead in the evening.
Plymouth - Good local operation and certain road trains can be very random (Which can be good or dull on any given day). Best times are 10am-2pm and 4:30pm-9pm on weekdays.
Delray - Best place for train volume and variety, but the neighborhoods around it are rough.
"...and I was in the front and Matt grabbed and pulled my ears from behind me and made horsey sounds."