Double Engine?

I am relatively new to the train world, so forgive my ignorance, but in the photo section, there is an image (http://railroad.net/photos/nycarea/media/ov-08.jpg) of two engines back-to-back pulling a metro north train on the harlem line towards wassaic. i always thought two engine were for really heavy loads, but this run is typically done by a single engine. can anybody enlighten me as to why there are two here?

The F units were only 1800 Hp and roughly rated for only 4 passenger cars each, MNCR runs 6 and 7 car trains, so two were needed.
These days same trains are handled by one P32acdm unit of 3200 hp

I followed the photo link you provided - you had failed to mention that the lead loco was painted in New York Central colors - I had seen this loco on a trip down to NYC from here in Poughkeepsie (POhh 'KPSee LAST STOP!) - this is kind of special - the date of the photo is 2002 so maybe it was a railfan trip? or maybe MNCRR ran out of working GE locos for this run? - pre-1995 MNCRR had to use the old DM FL9's and ocasionaly the SM F10's - #412 and #410 I have seen here in Poughkeepsie a long time ago in the pre- GE P32 Genesis -'Geny' era

in 2002 the F units were still used on upper Harlem and Danbury and Waterbury branch on shuttle trains.
They were no longer reliable enough to work as single units, the picture is front end of 3 car shuttle to Dover plains.