Statistics on freight are specified in terms of the weight of freight lifted, and the net tonne kilometre, being freight weight multiplied by distance carried. 116.6 million tonnes of freight was lifted in the 2013&ndash;4 period, against 138 million tonnes in 1986&ndash;7, a decrease of 16%. However, a record 22.7 billion net tonne kilometres (14 billion net ton miles) of freight movement were recorded in 2013-4, against 16.6 billion (10.1 billion) in 1986&ndash;7, an increase of 38%. Coal makes up 36% of the total net tonne kilometre, though its share is declining. Rail freight has increased its market share since privatisation (by net tonne kilometres) from 7.4% in 1998 to 11.1% in 2013. Recent growth is partly due to more international services including the Channel Tunnel and Port of Felixstowe, which is containerised. Nevertheless, network bottlenecks and insufficient investment in catering for 9 6" high shipping containers restrict growth.
Question: How many years did it take for rail freight to increase its market share by 3.7%?
Answer: 15