Income Taxes - Net Deferred Income Tax Provision (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Depreciation	$ 213	$ 212
Deferred grant income	(339)	(551)
Track maintenance credit	1,123	821
Accrued casualty and other claims	19	(25)
Accrued compensated time off and related payroll taxes	(17)	(19)
Share based compensation	(41)	(27)
Other (including the impact due to the change in the state income tax of $99 in 2013)	(167)us-gaap_DeferredOtherTaxExpenseBenefit	65us-gaap_DeferredOtherTaxExpenseBenefit
Change in valuation allowance	(852)	(257)
Net deferred income tax	$ (61)us-gaap_DeferredIncomeTaxExpenseBenefit	$ 219us-gaap_DeferredIncomeTaxExpenseBenefit

Deferred income tax accrued casualty and other claims.

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Deferred income tax accrued compensation time off related payroll taxes.

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Deferred income tax expense benefit depreciation.

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Deferred income tax share based compensation.

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Deferred income tax track maintenance credit.

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Deferred tax income tax change in valuation allowance.

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Amount of deferred income tax expense (benefit) and income tax credits.

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Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

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Amount of other deferred income tax expense (benefit) pertaining to income (loss) from continuing operations. For example, but not limited to, acquisition-date income tax benefits or expenses recognized from changes in the acquirer's valuation allowance for its previously existing deferred tax assets resulting from a business combination and adjustments to beginning-of-year balance of a valuation allowance because of a change in circumstance causing a change in judgment about the realizability of the related deferred tax asset in future periods.

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