* Siemens: winning contract could hurt existing commitments
* Siemens move leaves Bombardier, CAF, Hitachi as bidders
* Contract is worth 1 billion pounds
LONDON, July 5 (Reuters) - Siemens has pulled out of the bidding to provide trains for Britain's multi-billion pound Crossrail project, raising prospects for rivals Bombardier , Hitachi and Spain's CAF to win the contract.
Siemens said on Friday it no longer had the capacity to deliver 600 carriages for the new line to connect east and west London.
"Crossrail is a very large project and, since first undertaking our initial assessment of capacity and deliverability, Siemens has won multiple additional orders," the company said in a statement.
"To pursue another project of this scale could impact our ability to deliver our current customer commitments."
The German company said it would still deliver signalling and control systems for the Crossrail project.
Crossrail will link Heathrow west of London to the east of the city through huge new tunnels. Bids for the rail contract are due next month, with a decision expected by mid-2014.
Siemens withdrawal cuts the number of bidders for the 1 billion pound ($1.5 billion) train contract to three - Bombardier, Japan's Hitachi and Spain's CAF.
In 2011, Crossrail, Europe's largest infrastructure project, delayed the award of the contract to supply trains for the $26 billion project to save money. France's Alstom Transport - best known for building France's TGV high-speed trains - pulled out of the bidding two years ago.
Siemens recently beat Bombardier on a $2.4 billion contract to build 1,140 new carriages for use on the Thameslink rail line that runs through London and connects Bedford with Brighton on the south coast of England.
Bombardier was not immediately available for comment.