Here is a version that does not use array. Also, don't include the autonumber primary key in the INSERT action. If you need this PK as FK in OutDB, create a field to receive it.
Code:
Private Sub btnSplit_Click()
Dim rsOldData As DAO.Recordset
Dim strPre As String, Dim strLtrs As String, str2 As String
Dim i As Integer
Set rsOldData = CurrentDb.OpenRecordSet("SELECT * FROM OriDB")
Do While rsOldData.EOF = False
str2 = rsOldData![Line ID]
strPre = Left(str2, InStrRev(str2, "-"))
strLtrs = Replace(Mid(str2, InStrRev(str2, "-") + 1), "/", "")
For i = 1 To Len(strLtrs)
CurrentDb.Execute "INSERT INTO OutDB([Line ID], [Line Type], [Maintain Temp]) " & _
" VALUES('" & strPre & Mid(strLtrs, i, 1) & "', '" & rsOldData![Line Type] & "', " & rsOldData![Maintain Temp] & ")"
Next
rsOldData.MoveNext
Loop
rsOldData.Close
Set rsOldData = Nothing
End Sub
Code assumes every record in OriDB has a value in [Line ID] with appropriate structure.
Recommend no spaces nor punctuation/special characters (underscore only exception) in names.