Income Taxes - Net Deferred Income Tax Liability (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Deferred income tax liabilities - Differences between book and tax basis of property and equipment	$ 19,013	$ 18,802
Deferred income tax assets:
Deferred grant income	4,649	4,714
Track maintenance credit carry forwards	1,240	1,456
Alternative minimum tax carry forwards	75	75
Accrued casualty and other claims	59	146
Accrued compensated time off and related payroll taxes	245	261
Share based compensation	311	281
Allowance for doubtful accounts and other	72	101
Gross tax asset	6,651	7,034
Valuation allowance	(1,240)	(1,456)
Net deferred income tax liability	$ 13,602	$ 13,224