Wedding styles may change with seasons, but one of the things about gowns that remains constant is the train. And the guideline is - the length of the gown's train (and veil) determines how formal the gown is. Traditionally long, cathedral-length trains are best suited for an incredibly formal evening wedding. Shorter chapel or sweep trains are appropriate for a less formal daytime or evening wedding. 

There are several train styles to consider when selecting your gown. Trains may be attached and may require a bustle for ease of movement at the reception. Others are detachable for the reception. Here is a description of the various styles you may consider:

Royal - dramatic and extravagant train that falls up to twelve feet on the floor behind you. 
Cathedral - very formal length that falls between six and eight feet.
Chapel - the most popular choice for most brides, falls three to four feet.
Court - less formal and falls about a foot on the floor.
Brush or Sweep - barely brushes the floor.
Watteau - attaches to the shoulders rather than the waist.