Since I am preparing a clinic on prototype freight car chalk markings (not chalk graffiti) I decided to experiment with various marking tools for making white chalk marks in HO scale.
I used two fine point paint markers and three pencils. Displayed is a picture of:
Sharpie paint marker
Pen-touch paint marker
General charcoal white pencil
Prismacolor Verithin pencil
Supracolor II Soft pencil
Generally, these are available at Michael's, Aaron Brothers and Dick Blick stores or through their websites.
Displayed is a picture of an HO scale boxcar used for the experiment. The boxcar was given a coat of flat sealer before the marks were applied. The five marking tools were used in the order above, left to right.
I found the pencils easier to use than the paint markers and they did a better job of simulating chalk marks. The point of the General charcoal white pencil tended to crumble a bit so my recommendations are limited to the Prismacolor Verithin pencil and the Supracolor II Soft pencil.
I previously compared the Prismacolor Verithin pencil to a regular Prismacolor pencil and found the Verithin pencil gave better results so be sure to look for the Verithin version of that pencil brand.
All five of the marks were easily removed within five minutes using isopropyl rubbing alcohol and a cotton swab, so mistakes are easily eliminated.
If you want an alternative to these tools for making chalk marks, decals and dry transfers also can be used. I believe Clover House and Micro-Scale still make chalk marking decals.
I will post samples of prototype chalk markings in the near future.