Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
Central Trains
Table title:
Fleet at start of franchise
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=2 > Class </th><th colspan=1 rowspan=2 > Image </th><th colspan=1 rowspan=2 > Type </th><th colspan=2 rowspan=1 > Top speed </th><th colspan=1 rowspan=2 > Number </th><th colspan=1 rowspan=2 > Routes operated </th><th colspan=1 rowspan=2 > Built </th></tr>
<tr> <th colspan=1 rowspan=1 > mph </th><th colspan=1 rowspan=1 > km/h </th></tr>
<tr> <td colspan=1 rowspan=1 > Class 150 Sprinter </td><td colspan=1 rowspan=1 > 150121 at Kidderminster.JPG </td><td colspan=1 rowspan=1 > diesel multiple unit </td><td colspan=1 rowspan=1 > 75 </td><td colspan=1 rowspan=1 > 120 </td><td colspan=1 rowspan=1 > 29× 2 car 9× 3 car </td><td colspan=1 rowspan=1 > Some Centro services. Dorridge/Shirley/Stratford Upon Avon/Leamington Spa – Worcester/Great Malvern/Hereford. </td><td colspan=1 rowspan=1 > 1984–1987 </td></tr>
<tr> <td colspan=1 rowspan=1 > Class 153 Super Sprinter </td><td colspan=1 rowspan=1 > 153385 at Nottingham.JPG </td><td colspan=1 rowspan=1 > diesel multiple unit </td><td colspan=1 rowspan=1 > 75 </td><td colspan=1 rowspan=1 > 120 </td><td colspan=1 rowspan=1 > 21 </td><td colspan=1 rowspan=1 > Lincolnshire and Mid Wales rural services, Stourbridge branch line, Skegness to Crewe & Derby to Matlock and Nottingham to Birmingham New Street/Shrewsbury via Derby </td><td colspan=1 rowspan=1 > 1987–1988 </td></tr>
<tr> <td colspan=1 rowspan=1 > Class 156 Super Sprinter </td><td colspan=1 rowspan=1 > - </td><td colspan=1 rowspan=1 > diesel multiple unit </td><td colspan=1 rowspan=1 > 75 </td><td colspan=1 rowspan=1 > 120 </td><td colspan=1 rowspan=1 > 20 </td><td colspan=1 rowspan=1 > Aberystwyth and Chester to Birmingham New Street, Hereford to Birmingham New Street, Birmingham New Street to Leicester, Nottingham, Lincoln Central & Grimsby Town, Nottingham to Worksop, Crewe to Skegness. </td><td colspan=1 rowspan=1 > 1987–1989 </td></tr>
<tr> <td colspan=1 rowspan=1 > Class 158 Express Sprinter </td><td colspan=1 rowspan=1 > 158751 at Westbury.JPG </td><td colspan=1 rowspan=1 > diesel multiple unit </td><td colspan=1 rowspan=1 > 90 </td><td colspan=1 rowspan=1 > 145 </td><td colspan=1 rowspan=1 > 36 </td><td colspan=1 rowspan=1 > Norwich to Liverpool, Stansted to Birmingham, Nottingham to Cardiff, Grimsby to Birmingham, Birmingham to Liverpool </td><td colspan=1 rowspan=1 > 1989–1992 </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > Class 310 </td><td colspan=1 rowspan=1 > - </td><td colspan=1 rowspan=1 > electric multiple unit </td><td colspan=1 rowspan=1 > 75 </td><td colspan=1 rowspan=1 > 121 </td><td colspan=1 rowspan=1 > 10 </td><td class="highlighted"  colspan=1 rowspan=2 > Already being withdrawn at privatisation, remained as backup for the newly introduced Class 323. </td><td colspan=1 rowspan=1 > 1966 </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > Class 312 </td><td colspan=1 rowspan=1 > - </td><td colspan=1 rowspan=1 > electric multiple units </td><td colspan=1 rowspan=1 > 90 </td><td colspan=1 rowspan=1 > 145 </td><td colspan=1 rowspan=1 > 4 </td><td colspan=1 rowspan=1 > 1976 </td></tr>
<tr> <td colspan=1 rowspan=1 > Class 323 </td><td colspan=1 rowspan=1 > - </td><td colspan=1 rowspan=1 > electric multiple units </td><td colspan=1 rowspan=1 > 90 </td><td colspan=1 rowspan=1 > 145 </td><td colspan=1 rowspan=1 > 26 </td><td colspan=1 rowspan=1 > Cross City Lines </td><td colspan=1 rowspan=1 > 1992–1993 </td></tr>
</table>
Question:
What exactly happened to Class 310 and Class 312 trains?
Answer:
Class 310 and Class 312 trains were in the process of withdrawal at privatisation.