RESTRUCTURING - Schedule of Restructuring Charges (Details) - Integration 3.0 - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 4	$ 4	$ 7	$ 11
Corporate Activities and Elimination
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2	0	1	0
Freight Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	2	2	3
Transit Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2	2	4	8
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServicesSold \| Freight Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 0	$ 1	1	2
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Cost of Product and Service Sold	Cost of Product and Service Sold
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServicesSold \| Transit Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 2	$ 1	3	3
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Cost of Product and Service Sold	Cost of Product and Service Sold
Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense \| Freight Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 0	$ 1	1	1
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Selling, General and Administrative Expense	Selling, General and Administrative Expense
Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense \| Transit Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 0	$ 1	$ 1	$ 5
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Selling, General and Administrative Expense	Selling, General and Administrative Expense