Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your pups are chewing on and ingesting railroad ties used to anchor the canvas that is sheltering the back patio.
The short answer is that yes, eating railroad ties, and in fact even chewing on them is very dangerous for your pups and kitties. Animals often chew on treated wood, attracted by the taste, but it is very dangerous.
Railroad ties are protected from the effects of the elements by applying preservative products that contain creosote. Creosote is distilled from coal tar.
Phenol is the most dangerous toxic ingredient in coal-tar products. The approximate oral dose in which 50% of animals die after ingesting (LD50) of phenol for most species is 0.5 g/kg, or 0.22mgs per pound of body weight, except for cats which are more susceptible because they lack the ability to metabolize phenols and excrete them rapidly. Phenols are very irritating on contact, and ingestion results in ulcers and tissue death of the skin, lips, gums, tongue, esophagus and stomach. After ingestion or contact with the skin and absorption phenols accumulate in the liver and kidneys, commonly resulting in liver damage and kidney damage.
Cresols are another toxin present in rail road ties and are readily absorbed orally and through the skin. The oral lethal dose is 100–200 mg/kg, or 50-100mgs per pound of body weight, except in cats as I mentioned because they are more sensitive.
Cresols and phenols are locally corrosive, causing tissue death, ulcers, and scarring of skin, mouth, and esophagus, as well as central nervous system stimulation, tremors, and incoordination, and finally cardiovascular depression and shock. Capillary damage and kidney and liver damage can occur.
Death after ingesting a toxic amount can occur from 15 min to several days after exposure.
The first sign of poisoning often is several dead animals.
Due to liver damage we may see inability of the blood to clot properly, either slowly or not at all, and excessive fluid in the abdominal cavity.
There is no specific antidote for coal-tar/creosote product poisoning. Activated charcoal may reduce absorption.
Owners may reduce exposure by administration of egg whites to dilute and bind the phenols.
Skin exposure and irritation can be lessened by bathing with glycerol followed by liquid dish soap.
If organ damage occurs then hospital will be needed for supportive therapy for shock, liver and kidney damage, respiratory failure, and blood gas irregularities. Drug therapies such as N-acetylcysteine have been recommended.
Oral antibiotics, B vitamins, vitamin E, and high-quality–protein diets may aid recovery as well.
Of course the other concerns when chewing on and ingesting any wood are splinters and secondary abscesses in the mouth and throughout the gastrointestinal tract, peritonitis and a gastrointestinal blockage.
In short you absolutely should remove the ties and find another way to anchor the canvas, and you need to watch closely for any signs of toxicity and get your animals seen promptly should they occur.
Please let me know if you have any further questions.