Income Taxes - Net Deferred Income Tax Liability (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Deferred income tax liabilities - Differences between book and tax basis of property and equipment	$ 18,802	$ 18,647
Deferred income tax assets:
Deferred grant income	4,714	4,267
Track maintenance credit carry forwards	1,456	2,579
Alternative minimum tax carry forwards	75	75
Accrued casualty and other claims	146	164
Accrued compensated time off and related payroll taxes	261	244
Share based compensation	281	240
Allowance for doubtful accounts and other	101	101
Gross tax asset	7,034	7,670
Valuation allowance	(1,456)	(2,308)
Net deferred income tax liability	$ 13,224	$ 13,285