5 of the Lionel motorized units have a thin window strut that is easily broken.  3 others have multiple window struts that may still be broken. Many do not understand these units should not be picked up by grabbing the sides. Picking them up this way can break the window strut.  I even forget and start to pick these up from gripping the side.  
I have made a 3D printed brace that fits inside the shell and supports the side and window area.  One design for 51, 53, 56, 57 and 58 units and one for 41,42 and 59 units.

• There are reliefs for the lights.  The relief allows using an LED light for the rear light.
• The front and back top windows are not covered. This is to vent the heat from the motor.  I did not want to make a hot box that would cause other problems.
• There is a  draft on the sides to match the draft in the units. Many adjustments were made to fit the reliefs and supports on the inside of the body.
• There is clearance for the pulley and belt in the 58 unit.  
•  To assemble install in shell, then put the shell on the frame.   The light relief notch goes to the back of the shell.

Printed in black or dark gray it stays in the background.  Pictures of an interior could be pasted on the sides that cover windows to add detail to the engine. If this is done, adding an interior light and printing in a clear or translucent color  could add a lighted interior effect.

The files are in Millimeters.

These may work with some modern shells, but not with TMCC enabled modern reproductions. The modern shells have strengthened the struts.

vhubbard-window-brace-51.stl and .stp are for the 51, 53, 56, 57 and 58 units  Not all pictures show the final version with the notch in the back for pulley clearance.

Used with a 58 unit.   This rotary snow plow was not designed to run backwards, but if it is run backwards the armature will move up.  If more than .060”/ 1.5mm when reversing, the pulley may drag on the shell itself.  You will need to add spacer washers to the top of the armature, under the brush plate to limit the movement of the armature. 

vhubbard-windowbrace-41.stl and .stp  for 41,42 and 59 units. 3D printing
The top of the part is down against the print sheet. 0.2mm or smaller layers so it looks smooth in the window.  vhubbard-windowbrace-51 has 2 ledges that need supports. vhubbard-windowbrace-41.stl has one ledge that needs supports. 
The rest will print without supports when oriented as shown. 
This is a tight fit. These may print wider/narrower depending on the printer.  I used a Prusa MK4S.  Scale if needed.  .2mm/.008” may be the difference between dropping in and being too tight on the width.  

I used PETG.  PLA should also work well.

Free for private use.  All Rights reserved for commercial use.
These project .stl and step files I created are available on the OGR 3D printing Catalog.