generate cash without additional external financings . free cash flow should be considered in addition to , rather than as a substitute for , cash provided by operating activities . the following table reconciles cash provided by operating activities ( gaap measure ) to free cash flow ( non-gaap measure ) : millions 2014 2013 2012 .
what was the percentage change in free cash flow from 2013 to 2014?
2015 outlook f0b7 safety 2013 operating a safe railroad benefits all our constituents : our employees , customers , shareholders and the communities we serve . we will continue using a multi-faceted approach to safety , utilizing technology , risk assessment , quality control , training and employee engagement , and targeted capital investments . we will continue using and expanding the deployment of total safety culture and courage to care throughout our operations , which allows us to identify and implement best practices for employee and operational safety . we will continue our efforts to increase detection of rail defects ; improve or close crossings ; and educate the public and law enforcement agencies about crossing safety through a combination of our own programs ( including risk assessment strategies ) , industry programs and local community activities across our network . f0b7 network operations 2013 in 2015 , we will continue to add resources to support growth , improve service , and replenish our surge capability . f0b7 fuel prices 2013 with the dramatic drop in fuel prices at the end of 2014 , there is even more uncertainty around the projections of fuel prices . we again could see volatile fuel prices during the year , as they are sensitive to global and u.s . domestic demand , refining capacity , geopolitical events , weather conditions and other factors . as prices fluctuate there will be a timing impact on earnings , as our fuel surcharge programs trail fluctuations in fuel price by approximately two months . lower fuel prices could have a positive impact on the economy by increasing consumer discretionary spending that potentially could increase demand for various consumer products that we transport . alternatively , lower fuel prices will likely have a negative impact on other commodities such as coal , frac sand and crude oil shipments . f0b7 capital plan 2013 in 2015 , we expect our capital plan to be approximately $ 4.3 billion , including expenditures for ptc and 218 locomotives . the capital plan may be revised if business conditions warrant or if new laws or regulations affect our ability to generate sufficient returns on these investments . ( see further discussion in this item 7 under liquidity and capital resources 2013 capital plan. ) f0b7 financial expectations 2013 we expect the overall u.s . economy to continue to improve at a moderate pace . one of the biggest uncertainties is the outlook for energy markets , which will bring both challenges and opportunities . on balance , we expect to see positive volume growth for 2015 versus the prior year . in the current environment , we expect continued margin improvement driven by continued pricing opportunities , ongoing productivity initiatives and the ability to leverage our resources as we improve the fluidity of our network. .