CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income (loss)	$ 22,487	$ 2,803	$ (1,373)
Adjustment to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	6,635	3,792	2,395
Loss on disposal of asset	498		22
Provision for bad debt		131
Stock-based compensation	3,701	127	64
Amortization of debt issuance costs	51	4
Change in payables related to parties pursuant to Tax Receivable Agreement	(23,022)
Deferred income tax expense	25,652
Other	(28)
Changes in assets and liabilities:
Accounts receivable	(8,469)	(3,065)	1,047
Prepaid expenses and other assets	(3,273)	109	1,148
Inventories	(7,532)	327	1,794
Accounts payable	4,224	41	(1,515)
Accrued liabilities	5,805	252	(1,426)
Net cash provided by operating activities	26,729	4,521	2,156
Cash flows from investing activities:
Investment in property, plant and equipment	(93,912)	(10,899)	(27,790)
Proceeds from disposal of asset			4
Purchase price adjustment on acquisition			(73)
Cash paid for Railtronix acquisition	(5,000)
Investment in intangible assets	(72)	(36)
Net cash used in investing activities	(98,984)	(10,935)	(27,859)
Cash flows from financing activities:
Payments under capital leases	(27)	(25)	(25)
Payments under notes payable	(451)	(211)	(254)
Proceeds from borrowings under the credit facility	3,000	2,500
Repayment of credit facility	(5,500)
Payments related to debt issuance costs	(111)	(153)
Proceeds from members’ contributions		948	8,162
Payments to purchase member units			(5)
Proceeds from issuance of Class A common stock sold in initial public offering, net of offering costs	111,075
Proceeds from issuance of Class A common stock sold in November Offering, net of offering costs	44,684
Distributions paid to unitholders	(25,818)
Proceeds from pay down of promissory note related to membership units	5,256
Net cash provided by financing activities	132,108	3,059	7,878
Net increase (decrease) in cash	59,853	(3,355)	(17,825)
Cash at beginning of period	3,568	6,923	24,748
Cash at end of period	63,421	3,568	6,923
Non-cash activities
Capitalized depreciation in property, plant and equipment	668	674	539
Property and equipment additions incurred but not paid at year-end	7,765	264	198
Issuance of shares in acquisition	4,505
Notes payable issued for property, plant and equipment		397	297
Accrued interest from notes receivable issued for membership units		327	208
Cash paid for:
Interest	104	20	18
Income taxes	$ 45	$ 35	$ 60