Crime is down but some violence is up in the Big Apple’s transit system, according to NYPD data. A maniac repeat-offender pushed an elderly man to his death on Manhattan subway stairs Thursday, cops said — the fourth homicide in or around the subway this year, up from just one a year ago at this time. Cops brought Rhamell Burke, described as an “emotionally disturbed person,” to Bellevue Hospital’s psych ward at 3:30 p.m. Thursday. He was released an hour later and at 9:30 p.m. allegedly shoved