1000tintoys,
Clicker is basically correct about contrails. It all depends upon whether the atmosphere at any given level and time is of the right temperature and humidity to allow the water vapour produced by combustion of jet fuel to condense as visible water droplets. A bit like your breath condensing on a cold winter day. Meteorological conditions change from day to day, so there is no absolute contrail level, although certain level bands are more likely to produce conditions favourable for their formation.
As regards bunching of aircraft, I suggest you do a bit of googling for 'airways'.