This should do it:

Option Explicit

Sub CollateDatedData()
Dim fPATH As String, fNAME As String, NR As Long, fDate As String
Dim wb As Workbook, ws As Worksheet, wsMaster As Worksheet

On Error GoTo ErrorExit
Application.ScreenUpdating = False                  'speed up macro, turn off screen flicker
                                                    'make sure master has titles in row1 already
Set wsMaster = ThisWorkbook.Sheets("Master")        'edit to name of sheet for report
fPATH = "S:\Railserve\Availability\"                'remember the final \ in this string

If MsgBox("Clear the current data on the master?", vbYesNo, "Reset Master") = vbYes Then
    wsMaster.UsedRange.Offset(1).Clear              'option to clear data, keep titles
    NR = 2                                          'start entering data at row 2
Else
    NR = wsMaster.Range("A" & Rows.Count).End(xlUp).Row + 1     'add data to existing data, next row
End If
    
fNAME = Dir(fPATH & "Availability*.xlsx")           'get the first filename from the fPATH

Do While Len(fNAME) > 0                             'process each file one at at time
    fDate = Replace(Replace(fNAME, ".xlsx", ""), "Availability", "")
    fDate = Left(fDate, 2) & "/" & Mid(fDate, 3, 2) & "/" & Mid(fDate, 5, 2)    'create date string
    Set wb = Workbooks.Open(fPATH & fNAME)                                      'open the found file
    wb.Sheets("Car Summary By Product Line").Range("A4:I4").Copy                'copy the date
    wsMaster.Range("A" & NR).PasteSpecial xlPasteValues                         'paste values only
    wsMaster.Range("A" & NR).PasteSpecial xlPasteFormats                        'correct formatting
    wsMaster.Range("J" & NR).Value = fDate                                      'add date to column J
    wb.Close False                                  'closed the found file
    
    NR = NR + 1                                     'increment the next row for data
    fNAME = Dir                                     'get the next filename from FPATH
Loop                                                'repeat til no more filenames

ErrorExit:
Application.ScreenUpdating = True                   'return to normal speed, update screen
End Sub