I am not able to put this train set together some tracks have letters others numbers

hi Joyce.
is this a electric train set or a battery set?
could you please give me more info on this please.
can you please tell me what train set this is.
or go to this link might help you out on how to put a train set together.
how to put train track together Google Search

Have you cleaned your wheels & roller pickups?

Is the track properly and carefully put together? Is one or more sections damaged take one of the cars and gently push it around watching in those places where the train derails. If it only happens at a track joint look at it more carefully.

are you referring to the figure 8 piece?

Here is how Shake Shake bridge goes together:
On the bottom of each piece is a letter. Here is how the letters fit together to form an oval. Bridge is A and the other straight track is F you also have 2 B's & 2 C's

going clockwise: A - B - C - E - F - D - C - B
The 2 tall supports go under the junctions at B & C on either side and the small supports go under the junctions at C & D and C & E

On the bottom of each piece is a letter. Here is how the letters fit together to form an oval. Bridge is A and the other straight track is F you also have 2 B's & 2 C's
going clockwise: A - B - C - E - F - D - C - B
The 2 tall supports go under the junctions at B & C on either side and the small supports go under the junctions at C & D and C & E

use this as a reference
http://us.pegperego.com/toys-catalog/2011/Figure+8+Conversion+Track+Set

HOLD TWO PIECES OF TRACK TOGETHER SNAP ONE SIDE TOGETHER THEN SNAP OTHER SIDE.

CAT Iron Diesel Train Set's price decreased by 56% from $44.99 to $19.99 at Amazon.com. CAT Iron Diesel Train set. Easy assembly just push track together. Create multiple layouts. Requires 2 AA Batteries not included

Sale Price: $19.99, Vendor: Amazon.com Full Details
CAT Iron Diesel Train set. Easy assembly just push track together. Create multiple layouts. Requires 2 AA Batteries not included
Sale Price: $19.99, Vendor: Amazon.com and/or Amazon.com Merchants