Custom index in M language

I think the strategy here would be to Group By TractorNumber with all rows, then within each group use Groupkind.local to get the consecutive repeats.  By adding the Index at the right stage and expanding, it should work.

Here's the M code (and I wouldn't normally deliver everything but it is reasonably complex)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("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", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [TractorNumber = _t, Status = _t, Terminal = _t, StartDateTime = _t, #"Ranking (End Result)" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"TractorNumber", Int64.Type}, {"Terminal", type text}, {"Status", type text}, {"StartDateTime", type datetime}}),
    #"Sorted Rows" = Table.Sort(#"Changed Type",{{"TractorNumber", Order.Ascending}, {"StartDateTime", Order.Descending}}),
    #"Removed Columns1" = Table.RemoveColumns(#"Sorted Rows",{"Ranking (End Result)"}),
    #"Grouped Rows" = Table.Group(#"Removed Columns1", {"TractorNumber"}, {{"2ndAll", each _, type table [TractorNumber=nullable number, Status=nullable text, all=table]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each Table.Group(
                    [2ndAll]
                    , {"TractorNumber", "Status"}, {{"all", each _, type table [TractorNumber=nullable number, Status=nullable text, Terminal=nullable text, StartDateTime=nullable datetime]}}
                    , GroupKind.Local)),
    #"Added Index" = Table.AddColumn(#"Added Custom", "test", each Table.AddIndexColumn([Custom], "Index", 1, 1, Int64.Type)),
    #"Removed Columns" = Table.RemoveColumns(#"Added Index",{"2ndAll", "Custom"}),
    #"Expanded test" = Table.ExpandTableColumn(#"Removed Columns", "test", {"Status", "all", "Index"}, {"test.Status", "test.all", "test.Index"}),
    #"Expanded test.all" = Table.ExpandTableColumn(#"Expanded test", "test.all", {"Terminal", "StartDateTime"}, {"test.all.Terminal", "test.all.StartDateTime"})
in
    #"Expanded test.all"

Please test at your side and let me know how it goes.