As fires go, it was not the biggest one Tewksbury firefighters have battled this year. But it may be one of the costliest in terms of manpower.

According to Capt. Mike Sitar, firefighters spent approximately six hours out behind 820 Livingston St., Tuesday afternoon, dealing with a pesky fire nestled in a huge stack of wood railroad ties.

The ties are soaked with creosote, generating a thick, black smoke that could be seen for nearly two miles, said Sitar. The initial call came in just after 11 a.m.

According to Sitar, the stack of ties was approximately 50 feet long by 10-12-feet wide and at least 10 feet tall. The fire crawled deep into the pile, meaning that as ties were extinguished and removed, new hot spots had to be dealt with.

The ties are the property of Pan-Am Railways (formerly Guilford Rail System), of North Billerica. Pan-Am personnel were on scene helping to remove the ties. Railroad police were also on hand, participating in the investigation.

Sitar acknowledged that railroad ties stacked near the tracks are at risk of catching fire from sparks given off by the wheels of passing trains and said it was likely the cause of this fire. He said there are several such stacks all along the freight train and commuter rail tracks in Tewksbury.

State law requires that rail companies maintain clear and safe pathways on the tracks they own. Back in 2005, the small town of Westminster, in Central Mass., took Guilford Transportation to court to recoup money spent battling two brush fires directly linked to sparks from trains igniting brush and railroad ties left too close to the tracks. The parties reached an settlement in the case.

Pan Am officials could not immediately be reached for comment regarding the fire or the railroad ties.