CANdiy Shield

It is almost compatible with OpenLCB/LCC.
NB – Although pins 3 and 7 on the RJ45s should be joined together, pin 6 is not. This needs to be jumpered by a short wire between pins 6 and 7 on one of the RJ45s should do it.

NB – The voltage on the OpenLCB/LCC bus may exceed the voltage range permitted by the Arduino. Use caution if jumpering JP2.

NB – The RJ45s may not be compliant, as they may not be rated to carry 500 mA. However: “I purchased a few Watterott boards several months ago.  Looks like this is the RJ45 connector included with the board: https://www.te.com/usa-en/product-5555764-3.html?q=5555764&source=header#pdp-docs-features Here is the Product Spec document: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Specification+Or+Standard%7F108-1163%7FH%7Fpdf%7FEnglish%7FENG_SS_108-1163_H.pdf%7F5555764-3 It has a rating of 1.5A at 25C and 0.2A at 70C.  It looks like it can handle 0.5A up to about 65C.”

• Microchip CAN Controller (SPI)
  Microchip
• CAN Transceiver
  Switchable between 3V3 or 5V logic level (only CANdiy-Shield v2)
  
• 2 CAN connectors (RJ45)
  
• Prototyping Area
  
• Compatible with all Arduino Boards (SPI connection via 6-Pin ISP connector)
  
• Documentation on learn.watterott.com

Pins 4,5,6 are passed thru. But 6 is not connected to 3 and 7.
Can draw power from bus optionally, but see notes above.

See: https://github.com/watterott/CANdiy-Shield for schematics, etc.
Want to buy v2 at least.