saw a triangular shaped object flying low and slow following power lines on RR tracks at dusk

My self and a friend went into his backyard right at dusk- his house was on Northwest Highway and it runs parallel to the Chicago and Northwestern RR tracks. As we went into the backyard we spotted a triangular shaped object (dull white or grey)with a small light on each tip. It was maybe 12 to 14 feet long and perhaps 5-6 feet thick in the middle (it was tapered).
I saw it from full left to right; the timing was perfect. It was completely silent, and traveling about 60 feet off the ground following the power lines (from SE to NW) at about 10 knots. I had it in full view for what seemed an eternity- it was maybe 2 minutes.
I was 15 at the time and neither I nor my friend thought the houseful of people we just left would believe us so we kept it a secret.
Source of report indicates that the date of the incident is approximate.  PD