RESTRUCTURING - Schedule of Restructuring Charges (Details) - Integration 3.0 - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 2	$ 4	$ 9	$ 15
Corporate Activities and Elimination
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	2	1	2
Freight Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	1	2	4
Transit Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2	1	6	9
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServicesSold \| Freight Segment \| Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 0	$ 0	$ 1	$ 2
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Cost of Product and Service Sold	Cost of Product and Service Sold	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	$ 1	$ 0	$ 4	$ 3
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Cost of Product and Service Sold	Cost of Product and Service Sold	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	$ 2
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Selling, General and Administrative Expense	Selling, General and Administrative Expense	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	$ 1	$ 1	$ 2	$ 6
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]	Selling, General and Administrative Expense	Selling, General and Administrative Expense	Selling, General and Administrative Expense	Selling, General and Administrative Expense